Fixes #18955: Include the "clear" button only for non-required dropdown fields

[jeremystretch]

Fixes: #18955

• Enable the clear_button Tom-Select plugin only for fields without required set

[jeremystretch]

Need to determine why we're not setting the required attribute on required <select> fields.

[jeremystretch]

Need to determine why we're not setting the required attribute on required <select> fields.

AFAICT it's just not included in the template. We do use a custom template for the <select> widget, but it's not in Django's native template either. Feels like it should be, so I've added it.