Don't define our own user model class

Our own implementation was wrong because it specified the wrong user
collection (the default one called 'users'). I'm not sure what data
actually is in that collection, possibly it's historical data that's no
longer used by any app.

We need to store in a separate mongo collection defined by the
`USER_COLLECTION_NAME` constant.
Merge pull request #32 from alphagov/fix-user-authentication

Don't define our own user model class