[Events] Implement a block preventing setting two event registrations at the same time

[anjarfr]

What: If two or more committees set their event registration opening time at the same time, users will be ungrateful and complain because they can't sit ready to register for both events.
How: When an event is created, loop through all up-coming events which's registration has not yet started, and check if the registraion opening time is the same. If it is, a warning should be displayed, notifying the user that there is a collision.