Bugfix/Fix multiple display of completed items

[c-w]

As discussed in #172, there is an issue in the completed filter where each document is included once per annotation in the document. See screenshot below for an example where a document is included 4 times in the list:

The root cause of the problem is that the current filtering approach with {column_name}__isnull generates an inner join between the documents and annotations table which leads to the behavior of having one copy of the document per annotation.

This change fixes the behavior and ensures that each document is only included once in the completed list by using an explicit group by and count filter on the number of annotations:

Ideally we'd use queryset.distinct('id') instead of the group by count approach, but unfortunately this is currently only supported by Postgres.

Resolves #172