Add a debug view of the "Affected Path" index to Differential

Summary:
Ref T13639. The "Affected Path" table is currently hard to inspect: there's no UI, and using MySQL just gives you a bunch of IDs.

Add a simple UI and a debug-mode link to it.

Test Plan:

Maniphest Tasks: T13639

Differential Revision: https://secure.phabricator.com/D21615