* add a prosody module to insert identity information (when available) into
presence

prosody will check for jitsi_meet_context_user and
jitsi_meet_context_group in the session and, if they are present, insert
them into presence (we do this in prosody so they cannot be spoofed).

* remove unused 'presence' variable

* refactor to modify presence message in place

* make object member access consistent

* make the group information optional