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.
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