po plugin: redesign refresh mechanism after master page update

po plugin: redesign refresh mechanism after master page update

manually triggering IkiWiki::refresh() was at least dubious, and more or less
buggy (it randomly broke the whole backlinks feature); thinking a bit more to
add the necessary bits to @needsbuild seems like a better way. don't play with
ikiwiki's internals if not absolutely needed.

Signed-off-by: intrigeri <intrigeri@boum.org>