Implement SSO overriding avatars.

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.

lib/single_sign_on.rb

@@ -1,5 +1,5 @@
 class SingleSignOn
-  ACCESSORS = [:nonce, :name, :username, :email,
+  ACCESSORS = [:nonce, :name, :username, :email, :avatar_url, :avatar_force_update,
                :about_me, :external_id]
   FIXNUMS = []
   NONCE_EXPIRY_TIME = 10.minutes