a first Alembic revision file is joined to migrate existing databases to new mapping.
This patch:
- set `nullable=False` in various places
- drop legacy `message_types` table, and use `Enum` instead
- use naming convention, and get rid of anonymous constraint (this is the main reason
why the revision file is so long)
Alembic database migration tool, which is the recommended one for SQLAlchemy has been
integrated. When a database is created, it will be used to stamp to current (head)
revision, otherwise, DB will be checked to see if it needs to be updated, and upgrade will
be triggered if necessary.
