email_server: Add heading for manage aliases page

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
[fioddor: Keep email_alias.html based on email_form_base.html to avoid
inconsistent UI]
Reviewed-by: Fioddor Superconcentrado <fioddor@gmail.com>

plinth/modules/email_server/templates/email_alias.html

@@ -8,6 +8,8 @@
 
   {{ block.super }}
 
+  <h1>{% trans 'Manage Aliases' %}</h1>
+
   {% if no_alias %}
     <p>{% trans "You have no email aliases." %}</p>
   {% else %}