Document how to customize forms in ApplicationEditor
Ref T9132. I think the featureset is approximatley stable, so here's some documentation.
I also cleaned up a handful of things in the UI and tried to make them more obvious or more consistent.
Test Plan: Read documentation.
Reviewed By: chad
Maniphest Tasks: T9132
Differential Revision: https://secure.phabricator.com/D14718