Festival creation/edition -> redirect

[arca223]

Hello :)

When you create(add) a new festival or edit an existing one, you stay on the same form page.
(New -> Edit and Edit -> Edit)

It should redirect you either on the festival page (not existing yet) or on the discover page with map centered on the new festival to see it. A flashbag message should also be used to notify the user of the successful creation and edition

Priority of the feature choice :

1. Redirect to discover with map centered on the localisation of the new(or edited) festival with a flashbag (could be lot 2 of the choice below)
2. Redirect to discover with just the flashbag
3. Redirect the festival page

Maxime