Figure out an automatic DB migration plan for shared hosting

[aaronpk]

When installing an update on shared hosting, you won't necessarily have access to the command line to run the migrations.

We need to provide a way to run the migrations from the web UI when they're needed. This is a particular challenge because sometimes a migration is needed in order for the new code to not crash with DB errors. So maybe we catch DB errors and prompt to migrate the DB?