adjust to wikitcms changes in Pungi 4 compose handling
this is just a few changes needed to handle wikitcms' changes
to Pungi 4 compose handling. We're working off compose IDs, now,
rather than URLs, and we need to sort in a more sophisticated
(but unfortunately slower) way when doing testcase-stats to
account for the fact that nightlies and candidate composes may
now be interspersed (we can't really assume that nightly events
will only ever come before Alpha TC1 any more).