mattdrayer/api-currentcontentid: GET response returns current content identifier

@martynjames, as discussed here is support for returning the course content identifier -- take a look and let me know if any further changes are needed.
Ok, I've added in the "depth=2" performance tweak and confirmed that the unit tests are still passing, so it appears the field_data_cache approach is working as expected.  I also took a look at the DB queries in the debug toolbar enabled in the DRF web UI and I can see the singular FDC query in the list.  Take a look and let me know what you think.