Custom widget for handling large foreign key 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', '']
```

The issue names no files or tests; start by auditing the existing RESTful API and the admin foreign-key widget flow. Map the missing API pieces and the proposed _search_fields setting, with the work complete when large foreign-key results use the API and searchable fields are documented and configurable.

Written by the indexing model from the issue text.