(draft) Start introducing views perms

Old code for https://github.com/makinacorpus/Geotrek/issues/675

* [x] Generic decorator
* [x] From view to model permission name
* [x] Removed custom methods can_add() ...
* [x] Generate permissions on models automatically
* [x] Check permission for attachments
* [x] Internal user has read/export permissions
* [x] Redirect and show message instead of 403