Add project metadata to reversion Revision objects

This commit adds a supplementary metadata model which attaches the
project of versioned objects to a Revision. Additionally, it adds that
functionality to existing calls to reversion.create_revision() and
retroactively populates this field for past Revision objects.
Add new ElasticSearch activity index

The index is based off of reversion Version objects and is updated every
time a new Revision is made. In making the new index, I made a base
class for both it and the item index which includes common methods to
both.

Also deleted an empty file (editorsnotes/search/query.py)
Management command to rebuild activity index
New API view to view project activity

Based off editorsnotes.search.activity_index
Pull individual user activity from activity_index

instead of fetching from the DB each time