Summary:
Depends on D19279. Ref T13110. This implements the existing publishing logic for buildables, but does so via ModularTransactions instead of a core transaction type.
Since each application is implementing build transactions independently, this removes the core type.
Next, Differential will get a similar treatment.
Test Plan: Used `bin/harbormaster publish` (with some commenting-out-guard-clauses) to publish a commit Buildable; saw unchanged feed behavior.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13110
Differential Revision: https://secure.phabricator.com/D19280
