Autocomplete custom data function

This update allows the user to customise what data is sent in the AJAX call, enabling (for example) sending back data from other fields on the form. It follows on from the (minor) changes in #2305, which are included in this PR (they affect lines 37-39 of ModelAutocompleteType.php).

Added new `custom_data_function_block` option in ModelAutocompleteType
Added new `dropdown_auto_width` option in ModelAutocompleteType
Added related getDefaultOptions test and documentation
Updated form_admin_fields.html.twig to render effects of new options

I don't like the prefix `custom_`. Parameters are always custom if they're changed.