Add "Changes Planned" and "In Preparation" states for revisions
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.
Reviewed By: btrahan
Differential Revision: https://secure.phabricator.com/D8400