Merge: Opportunity

Addresses a few issues mentionned  in #811 :
- Return to the meetup creation page when the creation fails (auto-complete fields with old info)
- Use real buttons for the addition/deletion of an attendee to a Meetup
- Forbid creation of a Meetup without at least one answer

I also added feedback when an error is produced when creating the Meetup via a system similar to what @xymus does for Benitlux.

There were a few bugs hidden in the code, the update of the compiler with checks on useless null tests has revealed two of them.

Pull-Request: #850
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>