Implement user collections

Save a set of selected collections for later usage

Additional information:

Implementation:
x Implement model
o CRUD of user collections within Data Provider Administration (DPA)
o Assignment (an deassignment) of collections in DPA

(from redmine: issue id 34, created on 2018-01-22tgradl, closed on 2018-01-22)

• Relations:
  • relates #57 (closed)