Event creation form: enable edition

[Wardormeur]

Epic :

• https://github.com/CoderDojo/community-platform/issues/1230 Requires https://github.com/CoderDojo/community-platform/issues/1258 This means:
• editing an event created with the old UI (more than 3 ticket types fitting our default model) should redirect to the old UI
• cannot edit a published event (cc @conoramurphy, this is often asked to be changed by champs, but when booking as a parent, you're booking for what you see)
• cannot edit a past event
• cannot edit a canceled event
• anything should be fine