Enable chosen library on phone number fields

Alternate/similar idea: Integrate with Country → and Flags → and lean on those for our country select field.

Having tested Country a little, it doesn't directly provide the filtering functionality - that comes from Select2 Boxes → , which doesn't have a stable release.

So for our use case, I think we will:

1. Use the Chosen module →
2. Apply it to the existing select field - this could be done in a form alter, but I'd started investigating integration with the phone_number module, so I started MR34 - that patch makes it work, although ideally the change would declare dependencies etc.