Tor Messenger user agent (and other identifying strings)
We are setting the user agent and other identifiable bits like XMPP resource to an empty string. But with
CTCP, we are still revealing information about Tor Messenger (see #15161 (closed)).
We should decide how we want to identify Tor Messenger, whether by setting the user agent to an empty string, or setting it to some known product like Thunderbird or Instantbird.