The Avatar Settings form's clean_default_service() method attempted to

access cleaned data for other fields. Whether or not this works depends

on the order in which the Form class attempts to clean fields, and

broke in some manual testing I was performing, raising KeyErrors.
This switches this method to be a clean() method, allowing it to

operate on cleaned data from multiple fields and ensuring that saving

never breaks.

Saved the form many times without hitting these crashes.
Unit tests pass.