Presubmit to check that messages are updated

[nworden]

Since server_tests updates them when it runs, it used to be nearly impossible to neglect this, since they'd get updated as long as you ran the server tests locally before you opened your PR. However, with the Django migration, we're moving away from the current server testing framework and using Django's test framework instead. Overall that's been really nice, because the Django tests are way faster, but it does mean you can forget to update messages.