Migrations Tidy Up

[Joeyh021]

This was attempted in #187, but the schema generated was too different to the existing one on the production DB (difference checked using pgAdmin).

The advantages of doing this are:

• to remove old stuff
• totally dump the sqlalchemy_utils package as a dependancy (it is needed for decryption stuff)
• generate migrations with type stubs
• generate migrations that use the async dbapi (doesn't actually gain us much for migrations purposes)
• fix the slightly broken schema

The current best solution we can come up with is to write a script that connects to both the old and a new database and copies the tables over a row at a time.