Additional databases feature

This is for the `rewrite` branch.

Additional DBs allow custom buckets for stuff which can be useful in a number of situations.

To enable dbs, set this in the app config: `additional_user_dbs: ['foo', 'bar']`

This will create not only the regular user db `user/hoodieid` but also `user/hoodiid-foo` and `user/hoodieid-bar` with the same permissions as the regular user’s db.

Complete with tests.