fix: 🐛 Fix auth redirections

@smulquin This MR fixes a few bugs / instabilities:

• Buggy redirections to an event when user logs in or registers from an event page (dashboard flicking)
• Invalid redirection while updating language on user login
• Backend silent bug when a user is linked to an event but another link exists to the same event. This bug shows an error on frontend when we try to link an user to an event.