user: forbid manage of emails for external accounts.

- e.g LDAP accounts shouldn't allow to add additional emails.

rhodecode/templates/admin/my_account/my_account_emails.mako

@@ -47,8 +47,14 b''
           </table>
         </div>
 
+    % if c.user.extern_type != 'rhodecode':
+        <p>${_('Your user account details are managed by an external source. Details cannot be managed here.')}
+           <br/>${_('Source type')}: <strong>${c.user.extern_type}</strong>
+        </p>
+    % else:
         <div>
            ${c.form.render() | n}
         </div>
+    % endif
     </div>
 </div>