This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
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.
