Show an additional "Draft" tag on non-broadcasting revisions in a non-draft state

Summary:
Depends on D19284. Ref T13110. It's now possible to get a revision into a "Abandoned + But, Never Promoted From Draft" state. Show this in the header and provide the draft hint above the comment area.

Also, remove shouldBroadcast(). The method getShouldBroadcast() now has the same meaning.

Finally, migrate existing drafts to shouldBroadcast = false and default shouldBroadcast to true. If we don't do this, every older revision becomes a non-broadcasting revision because this flag was not explicitly set on revision creation before, only on promotion out of draft.

Test Plan: Ran migration; abandoned draft revisions and ended up in a draft + abandoned state.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19285