Add basic, manual migration handling

This adds a script to run "migration deploys". See the banner in-script for how
the process works (or how I think it'll work). It's not completely tested due
to our other deployment errors (slug too big).

I did test the script with the pg:psql and deploy lines commented, and that
worked. I also tested the individual migration scripts locally, and those
worked.

We need this now because, even if we sort out the slug size issue, the app
won't start in its current state without migrating the article -> articleURL
rename manually.
Should we put the app in maintenance mode instead?