Modularize the Differential "status" transaction and move away from ArcanistDifferentialRevisionStatus

Summary:
Ref T2543. Converts the TYPE_STATUS transaction (used to render "This revision now requires changes to proceed.", "This revision is accepted and ready to land.", etc) to ModularTransactions.

Also, continue consolidating all the status-related information (here, more colors and icons) into a single place. By the end of this, we may learn that NEEDS_REVIEW uses every color.

Test Plan:
Reviewed old status transactions (unchanged) and created new ones (looked the same as the old ones).

(I plan to migrate all of these a few diffs from now, around when I change the storage format.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18410