You can update the same "notifications" section that we allow you to
update in "my account".
This "subscriptions" section differs from the "my account" section
because we do not need to be logged in to update the status of the
notifications.
The user can access this page without being logged in.
We identify the user through the "subscriptions_token" parameter and
show a list of the notifications that can be enable/disable.
We will return a 404 error in case someone accesses the page with a
non-existent token.
We also control the case that some anonymous user tries to access the
page without any token, by returning the CanCan::AccessDenied exception.
