Custom widget for handling large many-to-many results

- Use the RESTful API!
- Document any missing pieces in the RESTful API.
- Determine API to set which field or fields will be searched against. Perhaps something like:

``` python
_search_fields = ['title', '']
```

Start by locating the existing admin widget and RESTful API entry points in the repository, then check which many-to-many and search-field behavior is already supported. Document any missing API pieces and define how a field-specific search configuration should work; done requires an agreed design and coverage for large many-to-many results.

Written by the indexing model from the issue text.