Use ElasticSearch to back the 0.4 /form/ API list endpoint

This is an incremental improvement, which adds a `/a/<domain>/api/form/` endpoint populated by ElasticSearch, supporting lucene queries, elastic search JSON queries, and simple field filtering, somewhat like the `case/` API.

This does not change the `/form/<id>` endpoint, so the two may be inconsistent, but I did not want to go into a major overhaul of anything.