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 support these applications.

Test Plan:
  - Ran migration.
  - Resolved "obsolete config" issue.
  - Viewed config setting.
  - Browsed prototypes in Applications app.
  - Viewed documentation.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley, hach-que

Maniphest Tasks: T6084

Differential Revision: https://secure.phabricator.com/D10493