SideEffects / ChatView: Add an unread cache.

This is to prevent wrong values in the stream.

lib/pleroma/web/views/streamer_view.ex

@@ -55,6 +55,8 @@ defmodule Pleroma.Web.StreamerView do
     # Explicitly giving the cmr for the object here, so we don't accidentally
     # send a later 'last_message' that was inserted between inserting this and
     # streaming it out
+    # 
+    # It also contains the chat with a cache of the correct unread count
     Logger.debug("Trying to stream out #{inspect(cm_ref)}")
 
     representation =