FIX: Chat summary email link in subfolder setups (#25188)

This regressed in 2791e75072. That commit
fixed subfolder URLs in general, but the `full_url` was adding the
subfolder prefix a second time, thus breaking this URL in emails.

plugins/chat/app/models/chat/message.rb

@@ -236,7 +236,7 @@ module Chat
     end
 
     def full_url
-      "#{Discourse.base_url}#{url}"
+      "#{Discourse.base_url_no_prefix}#{url}"
     end
 
     def url