Fix for #1782: select default language properly in facility_user template

* Add logic to `forms.py` to select language (works in most, but not all cases--not sure why; tested it's setting the right thing)
* Add JS to `facility_user.html` template to make sure that if a `default_language` dropdown value is not selected, the `DEFAULT_LANGUAGE` value is selected.

**Testing: Tested all 3 relevant scenarios:**
* Adding a new user
* Editing a user (`default_language=None`)
* Editing a user (`default_language` is set)