First stab at some location coverage.

This currently exposes a view, /api/location-coverage/ which spits out all Items, their terms, location and timestamp (is this what was being referred to as 'last feedback'? Unsure). Getting this up sooner rather than later to see if things are going in the right direction. I suppose this is just missing some 'export location coverage' button on the UI?

Using https://github.com/wq/django-rest-pandas as the library. Also used for Pula.