Add missing i18n

Add sort order by username in the user list
Add link to kiwiirc
the link could be replaced by a variable here to avoid translation typos.
See https://github.com/hotosm/osm-tasking-manager2/blob/e4c79773643bcf3f894f92a323ebdecbf2d9866b/osmtm/templates/task.current_locked.mako#L3 as an example.