Provide "Change Projects" and "Change Subscribers" (instead of "Add ...") in comment actions

Summary:
Ref T9908. Fixes T6205.

This is largely some refactoring to improve the code. The new structure is:

• Each EditField has zero or one "submit" (normal edit form) controls.
• Each EditField has zero or one "comment" (stacked actions) controls.
  • If we want more than one in the future, we'd just add two fields.
• Each EditField can have multiple EditTypes which provide Conduit transactions.
• EditTypes are now lower-level and less involved on the Submit/Comment pathways.

Test Plan:

• Added and removed projects and subscribers.
• Changed task statuses.
• In two windows: added some subscribers in one, removed different ones in the other. The changes did not conflict.
• Applied changes via Conduit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6205, T9908

Differential Revision: https://secure.phabricator.com/D14789