[Rich text editor] Add plain text mode

[jonnyandrew]

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Add a toggle to disable rich text editing and enter plain text mode.

Motivation and context

The plain text mode is a fallback for the rich text editor bypassing it completely.

Note that the plain text mode does not yet support markdown.

PSU-780

Screenshots / GIFs

	Before	After
Toggle
Toggle
Toggle off
Plain text mode

Tests

UI

• Change the text formatting setting
• Note the new setting is reflected in the UI

Message format

• Switch the text formatting setting on
• Note that messages are sent in HTML
• Switch the text formatting setting off
• Note that messages are sent as plain text

Editor

• Type a message
• Toggle the setting
• Note that the message is not cleared from the editor

Persistence

• Change the text formatting setting
• Restart the app
• View the text formatting setting
• Note the new setting is persisted

Tested devices

• Physical
• Emulator
• OS version(s): 33, 21

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

[jmartinesp]

LGTM! I don't like the idea of duplicating even more components, but the solution seems solid.

[jmartinesp]

This was already merged to the resilience-rc branch and that branch to develop, so we can close this PR.