R1:19bc91fd208d

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…