Move configuration of default locale and supported locales to instance config page in Admin Interface

[redshiftzero]

Description

For when #3211 is merged and deployed (since that'll happen in 0.8.0, then this ticket could be done at the earliest 0.9.0):

1. Add table in database instance_configuration
2. Store default locale and supported locales config values in there
3. Now to enable languages, no long Ansible playbook runs are required (./securedrop-admin install) and the admin has a merry UI to guide them

Other than admin UX, another reason to do this is that multiple Admins will never clobber one another's i18n/i10n related config variables. Generally the more config we move from site-specific on the Admin Workstation to the Admin Interface, we sidestep issues #2097.

User Research Evidence

The reason I think that this is worthwhile is because one of the frequent sources of admin support tickets and frustration are trying to use the securedrop-admin CLI to update one or two config values on the interface and having to deal with 1. Tor flakiness bailing the run and/or 2. having to sit and twiddle thumbs while the entire securedrop-admin install runs.

User Stories

As an Admin wanting to get documents in several non-English languages, I want a simple and user friendly way to enable support on my instance for some of the languages now supported by SecureDrop

[eloquence]

Since this is (as you correctly note) a common support pain point, I've added this to the 0.10.0 backlog. If we end up de-prioritizing it, we should at least add a section to the docs specifically about how to configure languages after the initial install (the admin guide on configuration would probably be the right place: https://docs.securedrop.org/en/release-0.8/admin.html#updating-system-configuration ).

[eloquence]

This won't make it to 0.10.0. Flagging to @edenemmanuel as potential docs task for the next sprint.

[eloquence]

(Discussed in backlog review today, still very much something we want to eventually support.)