New functionalities

I realized that there wasn't a function to delete events. So I implemented it. The user can select one or more events from a list with all the events inside of it and then delete them.

I made some changes to event models. I set name and date fields as unique_together, this is for not allowing users to create an event with the same name in the same date two or more times. As event names can be repeated, but in different dates, this is why I added this restriction. So, when the user tries to add a repeated event, a message is thrown informing the user the event already exists.

Also, I implemented the functionality that allows users to set the price of the ticket.