Make "first broadcast" rules for Differential drafts more general

Summary:
See PHI228. Ref T2543. The current logic gets this slightly wrong: prototypes are off, you create a draft with --draft, then promote it with "Request Review". This misses both branches.

Instead, test these conditions a little more broadly. We also need to store broadcast state since getIsNewObject() isn't good enough with this workflow.

Test Plan:

• With prototypes on and autopromotion, got a rich email after builds finished.
• With prototypes off, got a rich email immediately.
• With prototypes off and --draft, got a rich email after "Request Review".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18801