Fix mysql indexing validity in migrations

Currently, if you run syncdb and migrate from a fresh mysql db, it will fail on (among other things) the indexing of `accommodation_request` which is too long to be a mysql index.

This PR brings us one squashed bug closer to being able to run a dev environment (or any fresh environment) using mysql.

However, it involves modifying existing migration history which is one of the deadly sins. Is that worse than having a migration history which won't run on mysql? What will this PR do to environments with the existing history?

@brianhw What do you think? Who else should I ask to review this?