Backend: Units can be organized with Tags and Collections

[bradenmacdonald]

Make the backend changes required (if any) so that tags can be added to units, and so that units can be organized into Collections.

1. Verify that the existing "object tags" REST API can be used to apply tags to units (it should "just work" already thanks to this permissions code).

2. Write one or two new tests that tag units, and confirm that permissions are respected - include them in the content_tagging test suite.

3. Verify that units can be added to / remove from collections via REST API and that permissions are respected.

4. Write one or two new tests to cover this ^.