Begin moving Phabricator configuration into PHP

Summary: Ref T2255. Ref T2221. Lay the groundwork to move configuration into PHP, so we can show descriptions in the web UI, do typechecking, disable application options when an application is uninstalled, etc.

Test Plan:
{F28421}
{F28420}
{F28422}

Reviewers: codeblock, btrahan, vrana

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2221, T2255

Differential Revision: https://secure.phabricator.com/D4306