Do not expose IRC hostmasks in MUC participant metadata

Currently, biboumi puts IRC host names into MUC presences, e.g. (formatted for clarity):

<presence from='#channel%network@irc.zombofant.net/UserNickname' to='jonas@zombofant.net/e-Pria'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <status code='110'/>
    <item affiliation='admin' jid='~username@whatever-stuff-the-irc-has-here' role='moderator'/>
  </x>
</presence>

The jid attribute here has the nice advantage of showing the IRC host mask when a user joins. But it also is misleading to clients which assume that they can reach the entity under that JID. Some clients will for example prefer to start 1:1 chats using the bare JID as discovered from MUC presence if possible. Other clients may try to fetch an avatar and cause funny connection attempts on their XMPP server for no good reason.

I propose to either:

• Remove the jid attribute entirely or
• Replace it with the bare JID provided by biboumi to reach that user (in this case, usernickname%network@irc.zombofant.net)

Maybe the hostmask can be put into the resource, to still have it available to operators. This also has the advantage that the mapping from hostmask -> resource is less lossy than hostmask -> JID (think case-mapping).

(Will be happy to draft a MR once consensus is reached or @louiz has made a call.)