WIP improvements relating to postgresql storage setup
Fix DB relation
Using db_admin relation so we shouldn't care about what postgresql says the db name is.
Also make sure we create a db, not just init schema
Clarify readme relating to db storage.
Because it doesn't really have anything to do with this charm,
except if running a production service
Add a db-relation-departed hook.
It tries to drop the database, but unfortunately due to bug #8 it doesn't do it in time before PostgreSQL kills access. Some workaround needed - explained in README