The {% avatar_urls %} template tag looks up the avatar URLs for a user

and returns a JSON payload that represents them for use in a template.

Depending on the version of Python being used, the order of keys would

be different, making it annoying to unit test.
This change simply sorts the keys when dumping to JSON, giving us a

predictable order.

Unit tests pass on Python 2.7, 3.5, and 3.6.