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