Bug with signing up for concurrent sessions

Bug with sign up for events happening at the same time: one can sign up for several sessions happening simultaneously.

By defualt, if you try to sign up to events one by one, Crowdvine checks whether you're busy and disables control (removing "plus" button and replacing it with the notification "you signed up as attendee elsewhere") and everything is fine.

But if one opens several concurrent events in tabs and then clicks sign up buttons for each of them, Crowdvine doesn't check for potentional schedule conflict and adds those events to one's personal list.

Reproducing:
* Choose some concurrent events in the program;
* Make sure you aren't signed up for any of them;
* Open them in several browser tabs/windows;
* Try to sign up for all of them.