Improve automatic emails with temporary urls

This MR targets the improvement for automatic emails concerning validation of new users by supervisors/re-validation of users.

Fixes #542 (closed)

Fixes #550 (closed)

beat/web/ui/registration/templates/registration/mail.supervisor_validation_supervisee_add_request.message.txt

@@ -12,6 +12,9 @@ Username: {{ supervisee.username }}
 If you don't do this the supervisee will not be able to use his account in
 future and will not be recognized as your supervisee.
 
+You can also click on the following direct link to accomplish this action and accept this new supervisee
+at {{ prefix }}/accounts/validation/{{ temp_url }}
+
 If you are having problems to activate your supervisee account, contact a member of our
 support staff at {{ prefix }}{% url 'contact' %}.