Implemented by having Discourse download the image from the provided URL
and treating it as a custom upload.
Adds two more parameters to the SSO site’s response:
* `avatar_url` specifies the URL of the overriding avatar.
* `avatar_force_update` Discourse does not re-download avatars that
has already been download from the same URL. Setting this to true forces
Discourse to re-download the avatar in `avatar_url`
Note that both parameters are ignored if `sso_overrides_avatar` is set
to false.
