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
Add missing "are" to explanatory text

Summary: See D10493.

Auditors: btrahan
Provide an easy way to build phabricator/libphutil/arcanist rpm's to deploy
with a yum repo.

The rpm builds checking out the git repos. Version of the package is "0" and
the release contains a datetime (YearMonthDayHourMinute, UTC). Latest git rev
is saved to /opt/phacility/GIT-REVS

The RPM creates a daemon user (phabricator), log (/var/log/phabricator), data
directories (/var/lib/phabricator/{files|repo}) and a init script which manages
phd. Some default-settings are set if phabricator/config/local/local.json does
not exist.

Apache HTTPD and "phabricator/bin/storage upgrade" still have to be
configured/called manually.

The packages install the applications to /opt/phacility/{phabricator|libphutil|arcanist}.
Arcanist can be installed without phabricator webapp.

Build RPM with: rpmbuild -bb resources/rhel/phabricator.spec

TODO: start/stop service on upgrade, call "storage upgrade" on rpm upgrades