Fix KeyErrors when saving the Avatar Settings form.
Fix KeyErrors when saving the Avatar Settings form.

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.

Testing Done:
Saved the form many times without hitting these crashes.

Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/9404/

reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard