Tidying up against current state of Curate

* This adds migrations from Curate's engine
* Removes the VALID_CURATION_CONCERN_CLASS_NAMES which are now
registered.
* Removes _form which caused problems with underlying Curate works

Related to ndlib/curate#240
Related to #314
Merge pull request #57 from ndlib/planning-314

Tidying up against current state of Curate