Make DummyUser class inherit from ActiveRecord again.

This reverts changes from 7b1e66940ff744535469f0bbeb563adadfa68f13
Adds DummyUser factories.

The user factories where removed with extracting the user model into it's own gem. But is useful to have them for the dummy users.
Adds controller spec for locking page feature at page edit.
Do not raise not found error, if a page user could not be found. Fixes #560