Only register user into event if they fully fill the form

**Is your feature request related to a problem? Please describe.**
I have accidentally pressed the register button and are in few events i wasn't planning attending.

**Describe the solution you'd like**
Only register user to the event after the form is fully posted.

**Describe alternatives you've considered**
Nothing

Start by locating the event registration flow and the form submission handling in the repository, then trace what happens when the register button is pressed. The change is done when a user is registered only after the form is fully submitted, while an incomplete or accidental submission does not register them; add or update coverage for that behavior if the existing flow has tests.

Scritto dal modello di indicizzazione a partire dal testo della issue.