xnetevents: fix user count in event administration page.

There were several issues:
* the number of registered users was never displayed even if the phrasing
  suggested it.
* the number of lines in the 'all' view was broken because it contained
  the sum all the number of lines of all the 'moments' independently (as a
  consequence, the number of pages was always over-estimated).

Closes: #1655
Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>