DocMap filtering by reviewing service, date

[eidens]

Inspired by http://api.biorxiv.org/discussion_trip/help, the DocMap should have endpoints to search for DocMaps by reviewing service and/or date.

Searching by date should come in three flavours:

• Interval (start:end): return all DocMaps where a review happened within the given interval.
• Most recent (n): return the n DocMaps with the most recent reviews.
• Last days (d): return all DocMaps with a review in the last d days.

All queries should support offset-based pagination.

[eidens]

The queries by review service, interval, and n most recent days are implemented.

The query for the n most recent reviews requires some more restructuring of the code for creation of the Docmap output format. That part, going from the internal Docmap format to the one that is given out to users, should probably be implemented in the flask app anyway.