[FEATURE] Add new confirm and cancel registration actions with verification step

[derhansen]

Idea is to add 2 new actions to the EventController, which have the same signature as the current confirmRegistrationAction and cancelRegistrationAction. The new actions will not directly confirm/cancel the registration, but show a confirmation form. When the confirmation form is submitted, the user is redirected to the existing confirmRegistrationAction or cancelRegistrationAction with the previusly received $reguid and $hmac.