Tell omniauth which redirect_uri to use

As seen here:
omniauth/omniauth-oauth2#93
https://docs.microsoft.com/en-us/graph/tutorials/ruby?tutorial-step=3

This way we can have multiple callback urls setup in Azure AD.

app/lib/omni_auth/strategies/azure_ad_auth.rb

@@ -53,7 +53,7 @@ def user_roles
       # query parameters. Azure fails validation because that doesn't match the
       # registered callback.
       def callback_url
-        options[:redirect_uri]
+        options[:redirect_uri] || (full_host + script_name + callback_path)
       end
     end
   end