[REM] mail: remove follow / unfollow buttons in notification emails

Notification emails contain a link allowing to follow or unfollow
documents directly from the notification email. However there are
several issues with this button

 * it require people to login to follow or unfollow documents;
 * it is not linked to the email;
 * if the customer does not have a login it is not possible to perform
   the action;
 * there is no confirmation step;

This commit removes the follow / unfollow button in the notification
emails. Indeed as people have the "Access document" button there is
only one more click to perform in order to subscribe or unsubscribe.
Concerning customers adding a complete control of the follow mechanism
would be costly in term of code and access rights checks. We therefore
decided to remove this button.

It also allows to simplify a bit the notification process as there
are less parameters to take into account when grouping recipients. It
allows to lessen the number of required template rendering when sending
notification emails.

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