Make sure field filters are sorted before checking them
Moved search method into base search backend
Added search method to PageQuerySet
Implemented __in lookup in ElasticSearch backend

Also added an error message if a user attempts to use a subquery with the __in lookup.
Added get_indexed_objects method to Indexed class
This gives developers control over the QuerySet used when the model is added to the indexed
Use get_indexed_objects to speed up indexing of Images/Documents
Previously, this created a query for every single image and document to get the tags. This was very slow on RCA which has over 15000 images.
This commit fixes this by adding a prefetch_related to the QuerySet used for indexing.