R1:298604c9d35d

Rename "beta" to "prototype" and document support policy

Summary:
Fixes T6084. Changes:

- Rename `phabricator.show-beta-applications` to `phabricator.show-prototypes`, to reinforce that these include early-development applications.
- Migrate the config setting.
- Add an explicit "no support" banner to the config page.
- Rename "Beta" to "Prototype" in the UI.
- Use "bomb" icon instead of "half star" icon.
- Document prototype applications in more detail.
- Explicitly document that we do not…