Fix "personal" face in jabber-activity

We can't distinguish "personal" vs "non-personal" in
jabber-activity-mode-line-update, since that loops over _all_ entries in
jabber-activity-jids each time, and thus the JIDs will be marked either
all personal or all non-personal.  Instead, let's keep a separate list,
jabber-activity-personal-jids, which is updated when activity is logged
for a JID.