thread: Fix a couple issues with email message-ids

  * Fix matching of collaborators on return emails. Previously the
    collaborator ID was used instead of the related user ID. Also, the 'C'
    user class (collaborator) was not handled in ::decodeMessageId()
  * Only send a reply separator for email related to a thread
  * Always generate a tag for the email message id. This will allow reply
    from the help desk administrator for the new ticket alert