When a component's status changes or when an incident is
created/updated, an email is sent to every people that has subscribed.
At the end of the mail a link to manage its subscription has been
introduced.
Now, it uses the translation.
Untill now a subscriber wasn't able to manage its subscription because
there was no link anywhere, so it was forced to unsubscribe and then
subscribe again.
Now the link is in the notification email, after 'Unsubscribe at'.
Related to #2887