po: redesign (once more) automatic POT/PO files update

po: redesign (once more) automatic POT/PO files update

Now use the change hook to update these files, check them into VCS, and trigger
IkiWiki::refresh as needed. The needsbuild hook's help was required to prevent
infinite looping.

This more rigorous way of doing this fixes recentchanges (that was previously
not updated in some cases), and probably is a better long-term solution than the
two previously tested ones.

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