Give "Config" a full-width, hierarchical layout

Summary:
Depends on D20933. Ref T13362. This reorganizes Config a bit and attempts to simplify it.

Subsections are now in a landing page console and groupings have been removed. We "only" have 75 values you can edit from the web UI nowadays, which is still a lot, but less overwhelming than it was in the past. And the trend is generally downward, as config is removed/simplified or moved into application settings.

This also gets rid of the "gigantic blobs of JSON in the UI".

Test Plan: Browsed all Config sections.

Maniphest Tasks: T13362

Differential Revision: https://secure.phabricator.com/D20934