Partially decouple revision broadcasting from revision draft state

Summary:
Depends on D19283. Ref T13110. To enable "Changes Planned + But, Still A Draft" and "Abandoned + But, Never Promoted From Draft" states, decouple the "broadcast" flag from the "draft" state.

Broadcast behavior is now based only on the shouldBroadcast flag, and revisions in any state may have this flag.

Revisions gain this flag when created as a non-draft, or when they leave the draft state for the first time.

There are probably still some ways you can get the wrong result here -- maybe abandon + update -- but those can be cleaned up as they arise.

Test Plan: Kinda poked it a bit but I'll vet this more heavily at the end of this sequence.

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19284