Add common permission for sending user feedback

there can be valid reasons to *not* expose that, so let admin decide

tailbone/templates/themes/falafel/base.mako

@@ -330,9 +330,11 @@
             % endif
 
             ## Feedback Button / Dialog
-            <feedback-form
-               action="${url('feedback')}">
-            </feedback-form>
+            % if request.has_perm('common.feedback'):
+                <feedback-form
+                   action="${url('feedback')}">
+                </feedback-form>
+            % endif
 
           </div><!-- level-right -->
         </nav><!-- level -->