Add project field on all DRF serializers

Also make that field nested, so that it includes the name of the project
as well as the URL.
Update ES put_mapping handling

1. Make sure that mappings are actually put in the first place
(whoopsie). Now, mapping are put in place after a document is cleared &
being rebuilt.

2. Map topic name & project name document fields as not analyzed-- they
are used as facets and not really usefully searched in analyzed form.
Use ES for facted browsing of notes/documents

There's a lot of room for improvement here-- I've basically just plopped
ES in where Xapian used to be. I think it would make sense for both
these pages to be totally javascript-driven, since they are inherently
dynamic and would be well-suited for a backbone view.
Remove unneeded imports in main.views.documents
Add last_update field to DRF serializers
Import models in search as main_models
Deserialize zotero_data strings as dicts

This way, we can search them in ES
Refine topics serializer, add topics to ES index
Merge pull request #160 from editorsnotes/REST-api

Rest api