Force a specific event_rsvp_date with an hook (again)

As https://wordpress.org/support/topic/force-a-specific-event_rsvp_date-with-an-hook/

The patch wasn’t enough so I had to change in various places the code to be sure that the plugin take the right event_rsvp_date.

Our needs were the last day of the event +1 months as people forgot to mark their participation.
Diff on https://github.com/mozilla/community-portal#plugin-patched