Automate user creation

[Pivotal story](https://www.pivotaltracker.com/story/show/55989158)

Store user permissions in the database. This is very similar to the story to automate the creation of buckets #151.

- Create a `UserConfig` domain object and associated repository
- Integrate it with the write api
- Update the script to create seed file from current configuration
- Update the task to load the seed files
- Add a task to give a user access to a bucket