Add a UI for reviewing database schemata

Summary:
Ref T1191. Plan here is:

• Build a tool showing the current schemata status (this diff).
• Have it compare the current status to the desired status (partly here, mostly in future diffs).
• Then add a migration tool, and eventually a setup issue to tell people to run it.

Test Plan:
Reviewed current schemata.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10494