Adding and deleting tickets

I recently finished the adding tickets part. I had to simplify it: by now if an user inserts a range of tickets which already exists, it overwrites them, making them to loose their previous values. I'm looking forward to improve this functionality later, by validating some other cases.

Also I added a new functionality, which is deleting tickets. The user may select (from a multiple choice field) which tickets are needed to be deleted.

I had a hard time on sending parameters to a form. One of the problems is that whenever a form receives a parameter (like eventId, for example), the is_valid method returns that the form is invalid, so nothing is done.