Add an explcit "Changes Planned" state for Differential

Summary: Ref T2222. Ref T4481. This fixes the issue where "Plan Changes" could immediately trigger a state change (e.g., back to accepted) because of state-based transitions out of the NEEDS_REVISION state.

Test Plan: Planned changes an "accepted" revision, it didn't immediately return to being accepted.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222, T4481

Differential Revision: https://secure.phabricator.com/D8403