Add switch to HTML editor

Allow users to switch between the Block editor and source (HTML) editing.

Related: https://meta.trac.wordpress.org/ticket/6608#comment:9

> I think there should be an option to fill out a simple form. In my professional opinion, I think Gutenberg still requires a bit too much of a learning process to really get the hang of it as a keyboard user. Screen readers only further complicate this. Since there is also no options menu, there is no keyboard shortcuts list or any helpful guides for new users to WordPress. I am not sure this is the experience we should be giving users who are new to WP wanting to ask their questions.

And: https://meta.trac.wordpress.org/ticket/6608#comment:57

> I prefer to write in plain HTML [...] -- please give power users the option to switch between blocks and HTML.

The issue names no files, tests, or entry points. Start by mapping where the Block editor is integrated into WordPress comments, admin pages, bbPress, and BuddyPress, then determine the intended switch behavior and accessibility requirements. Done means users can move between block and source HTML editing in the supported contexts.

Written by the indexing model from the issue text.