pundit: convert user name change requests to pundit.

Fix discrepancy between index action and show action. The index
action allowed members to see name changes for undeleted users, but the
show action didn't.

app/views/static/site_map.html.erb

@@ -121,7 +121,7 @@
         <% else %>
           <li><%= link_to "Profile", profile_path %></li>
           <li><%= link_to "Settings", settings_path %></li>
-          <% if CurrentUser.is_gold? %>
+          <% if policy(UserNameChangeRequest).create? %>
             <li><%= link_to "Change name", new_user_name_change_request_path %></li>
           <% end %>
           <li><%= link_to "Delete account", maintenance_user_deletion_path %></li>
@@ -150,7 +150,7 @@
         <li><%= link_to("Jobs", delayed_jobs_path) %></li>
         <li><%= link_to("Bulk Update Requests", bulk_update_requests_path) %></li>
 
-        <% if CurrentUser.is_member? %>
+        <% if policy(UserNameChangeRequest).index? %>
           <li><%= link_to("User Name Change Requests", user_name_change_requests_path) %></li>
         <% end %>