The default name of the database ("mediagoblin") was coded
in db/open.py and init/celery/__init__.py.
Instead use the new config system to have this as the
default for the "db_name" config option.
I needed to split the db connection/opening into open.py,
due to an import loop:
- util.py needs db/util.py:ObjectId
- db/util.py would need db/models.py
- db/models.py needs util.py:slugify