Support filtering on related fields

[gasman]

Fixes #12

Fix the validation phase of BaseSearchQueryCompiler._process_filter so that it correctly descends into RelatedFields definitions when confirming that a field being filtered on has a corresponding FilterField. (Previously it only looked in the search_fields definition of the base model of the query, giving spurious "Please add index.FilterField" errors.) This is sufficient to make filtering by related fields work for the database search backends (because these work by annotating and filtering the original query, retaining all existing filtering logic).

For Elasticsearch-based engines, we also need to rewrite the filter clause into Elasticsearch query syntax: where a filter on a field of the base model looks like:

{"term": {"number_of_pages_filter": 320}}

the equivalent for a field on a related model is:

{"nested": {
  "path": "authors",
  "query": {"term": {"authors.date_of_birth_filter": "1892-01-03"}}
}}

To achieve this, we revise the API of _process_lookup (which is only implemented by the Elasticsearch backends) so that it receives the full chain of RelatedFields definitions leading up to the FilterField being filtered on, and accepts a mapping parameter so that it can work with the mapping of any model (rather than just self.mapping which is tied to the query's root model). We can then call _process_lookup recursively to build the inner {"term": ...} clause, and wrap it in the {"nested": ...} clause.