Add "Changes Planned" and "In Preparation" states for revisions

Summary:
Ref T2222. Discussion in T4481. Ref T2543.

Primarily, this will let me fix some of the rough edges that came out of ApplicationTransactions and state-based transitions.

I'm also adding a constant for T2543 while I'm in here.

Test Plan: Used `grep` to look for any weird special behavior, didn't see any.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2543, T2222

Differential Revision: https://secure.phabricator.com/D8400