* Make FixBrokenGoals migration idempotent

The migration in question was renamed in order to fix order of executing migrations when run
from the ground up (via https://github.com/plausible/analytics/pull/3378).

As a side effect, it's executed again on databases that had it applied earlier, with
a different timestamp prefix.

As this migration is safe to run multiple times, it was modified to make forward
migration work gracefully when constraint already exists.

* Add `pending-migrations.sh` release script