R1:b48d4fabaf9e

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…