Reader - fix domain free plan upsell banner.

[Addison-Stavlo]

Related to # #101295

Proposed Changes

Fixes two issue with the banner:

1. The banner would disappear if there was no site currently selected in state. I updated the upsell nudge component to fall back to the most recently selected site Id and then primary site id if no site is currently selected.
   • Unlike most places upsells exist, the reader can be loaded in a state where no site is currently selected. However, it "feels like" a site is selected due to its appearance in the top admin bar. The top admin bar displays the site by the same fallback logic. Updating this component to fallback to these other site ID values allows it to work consistently in places like the reader, without changing its behavior in contexts were individual sites are always selected.
2. We update the banner to link to the specific flow build for this upsell, instead of the plans page for the site.

BEFORE

When clicked, the banner would land at:

AFTER

When clicked, the banner will land at:

Why are these changes being made?

1. We don't want the banner to disappear when it shouldn't. This limits opportunities for users to get the promotional upgrade, and our ability to upsell.
2. When it does work, we want it going to the newer flow that gives a better user experience for the upsell.

Testing Instructions

• Create a new account with 1 site on a free plan.
• Visit the reader, you should see the upsell banner.
• Click "W" in the masterbar to navigate to /sites.
• Without clicking anything else on /sites, click "Reader" in the masterbar.
• Verify the upsell is still present. This is where it would previously disappear.
• Click on the upsell and verify you are sent to the multi-step flow for this promotion. previously this was the plans page.

For testing the extra change/case around replacing the getSelectedSiteSlug selector with getSiteSlug, you can verify that these both provide the same return value when a site is selected.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-139099

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-139099&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-139099&env=a8c-for-agencies

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~344 bytes added 📈 [gzipped])

name                             parsed_size           gzip_size
themes                                +125 B  (+0.0%)      +26 B  (+0.0%)
staging-site                          +125 B  (+0.0%)      +30 B  (+0.0%)
site-settings                         +125 B  (+0.0%)      +27 B  (+0.0%)
settings-security                     +125 B  (+0.0%)      +34 B  (+0.0%)
settings-podcast                      +125 B  (+0.0%)      +33 B  (+0.0%)
settings-performance                  +125 B  (+0.0%)       +7 B  (+0.0%)
settings                              +125 B  (+0.0%)      +27 B  (+0.0%)
posts-custom                          +125 B  (+0.0%)      +32 B  (+0.0%)
posts                                 +125 B  (+0.0%)      +32 B  (+0.0%)
plugins                               +125 B  (+0.0%)      +24 B  (+0.0%)
overview                              +125 B  (+0.0%)      +27 B  (+0.0%)
marketing                             +125 B  (+0.0%)      +30 B  (+0.0%)
jetpack-cloud-plugin-management       +125 B  (+0.0%)      +24 B  (+0.0%)
hosting                               +125 B  (+0.0%)      +27 B  (+0.0%)
earn                                  +125 B  (+0.0%)      +37 B  (+0.0%)
domains                               +125 B  (+0.0%)      +38 B  (+0.0%)
activity                              +125 B  (+0.0%)      +26 B  (+0.0%)
a8c-for-agencies-plugins              +125 B  (+0.0%)      +24 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~307 bytes added 📈 [gzipped])

name                                                     parsed_size           gzip_size
async-load-calypso-blocks-jitm-templates-sidebar-banner       +291 B  (+0.6%)      +68 B  (+0.4%)
async-load-calypso-blocks-jitm-templates-notice               +291 B  (+0.6%)      +70 B  (+0.4%)
async-load-calypso-blocks-jitm-templates-default              +291 B  (+0.6%)      +68 B  (+0.4%)
async-load-calypso-reader-sidebar                             +160 B  (+0.2%)      +44 B  (+0.2%)
async-load-design-blocks                                      +125 B  (+0.0%)      +32 B  (+0.0%)
async-load-calypso-my-sites-current-site-notice               +125 B  (+0.2%)      +25 B  (+0.1%)
async-load-calypso-components-web-preview-component           +125 B  (+0.0%)      +32 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• blaze-dashboard
• notifications
• odyssey-stats
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/reader-domain-plan-upsell-banner on your sandbox.

[DustyReagan]

Tests well and makes sense to me.