copied and modified slightly from @ormsbee's #131

to associate PublishableEntities with Collections.

Collection.modified timestamp is updated whenever its contents are changed.

Addresses PR review.

* CollectionObject -> CollectionPublishableEntity,
  with added created_by and created fields
* contents -> entities, contents_qset -> entities_qset
* get_object_collections -> get_entity_collections,
  with added learning_package_id argument

We ended up not needing a "get all collections" function; we always
fetch them for a given learning package.

So we merged them into a single api.get_collections method that takes a
`learning_package_id` + optional `enabled` (defaults to True).

API:
* create_collection -- removed entities_qset param, added enabled param
* add/remove entities from a single collection, not a qset
* raise ValidationError if adding entities to a collection with
  mismatching learning packages
* add/remove returns updated collection object (not count)

Tests:
* use authoring API
* simplified data used by test classes

being added to a collection