remove the IsProviderMixin and introduce 3 new mixins:
- UserAccessMixin     -> must be a registered user
- ProviderAccessMixin -> user must be a provider
- AllAccessMixin      -> may or may not be a registered user

All these 3 mixins will also ensure that the user email is validated.

The purpose of the AllAccessMixin is to force the validation of the
email when the user is registered, thus the validation will be
requested when landing on the webapp_detail page rather than when
submitting the first job (which would be discarded)