Carve out a separate "Modules/Extensions" section of Config

Summary:
Ref T13362. Config is currently doing a ton of stuff and fairly overwhelming. Separate out "Modules/Extensions" so it can live in its own section.

(This stuff is mostly useful for development and normal users rarely need to end up here.)

Test Plan: Visited seciton, clicked around. This is just a visual change.

Maniphest Tasks: T13362

Differential Revision: https://secure.phabricator.com/D20930