Merge the "HarbormasterBuildCommand" table into "HarbormasterBuildMessage"

Summary:
Ref T13072. These two similar tables don't make sense to keep separate. Instead, make Build a valid receiver for BuildMessage objects.

These tables are practically the same, so this is straightforward: just copy the rows in and then drop the old table.

(This table was trivial and ephemeral anyway, so I'm not bothering to do the usual "keep it around for a couple years just in case".)

Test Plan:

• Populated BuildCommand table, ran migration, saw Builds end up in the proper transitional state (e.g., pausing, aborting, restarting) with appropriate queued messages.
• Queued new messages by clicking UI buttons.
• Ran BuildWorkers, saw them process messages and mark them as consumed.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21684