Some bug fixes, tweaks, and features

Please see the individual commit comments for details.  The first commit's avatar_url (bug fix) was re factored in a latter commit when i discovered thumbnail_exists..  The mods to the change view make it possible to mark an avatar as primary, upload a new image, make an avatar from a url, and delete an avatar form one place.  Also, added ajax support and an the ability to not make an avatar primary on upload/creation.