Change update game form to prevent switching a game from an event to another

[waliens]

Currently when a user tries to switch a game from an event to another, he is displayed a text error message after submitting the form. Maybe a better option would be to adapt the form in case an event is associated with the game: the event would be displayed as static text with a cancel cross. When the user clicks on the cross, a request is sent to dissociate the event and the form changes to its current state where the user can pick an event.
Alternatively, clicking on the cross would not trigger the update but the update would occur only when the user submits the form.