Add callback_with_admin option to ModelAutocompleteType

Options and test added to ModelAutocompleteType classes, handler added to HelperController and related documentation added in form_types.rst.

This work is intended to be combined with #2311 so you can customise the data function, send some extra parameters in the AJAX request and then pass the admin through to a callback_with_admin method which can use the extra request data to update the QueryBuilder and modify the results in the datagrid.