po: remove all dependencies on nicepagetitle

po: remove all dependencies on nicepagetitle

... as my meta branch probably won't be merged before the po plugin is, contrary
to what I was originally supposing.

This implies removing the po_translation_status_in_links and
po_strictly_refresh_backlinks options.

Added a note to the TODO section to think of bringing these features back later,
as they really enhance user experience on a translatable wiki.

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