Rename the Differential "hasBroadcast" flag to "shouldBroadcast"

Summary:
Depends on D19282. Ref T13110. I want to introduce "Changes Planned + Still A Draft" and "Abandoned + Still A Draft" states, at a minimum.

I think the "hasBroadcast" flag is effectively identical to a hypothetical "stillADraft" flag, so rename it to "shouldBroadcast" to better match its intended behavior.

This just changes labels, not any behavior.

Test Plan: Grepped for hasBroadcast and HAS_BROADCAST.

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19283