Validate uid tokens for unsubscription views

[Souleymane-T]

For unsubscription views here: https://github.com/Netsach/concrete-datastore/blob/master/concrete_datastore/concrete/views.py#L50

A get_object_or_404 is done without checking the type of the token. If it's not an UUID, an error is raised and it should only return a 400.

Suggestions:

• Check in the urls directly that we have an UUID in the path
• Add an UUID validation before the get_object_or_404