allow fast server to be disabled by removing it from the config file

app/views/domains/_nav.html.haml

@@ -1,4 +1,5 @@
 .navBar.navBar--secondary
   %ul
     %li.navBar__item= link_to "Domains", organization_server_domains_path(organization, @server), :class => ['navBar__link', active_nav == :domains ? 'is-active' : '']
-    %li.navBar__item= link_to "Tracking Domains", organization_server_track_domains_path(organization, @server), :class => ['navBar__link', active_nav == :track_domains ? 'is-active' : '']
+    - if Postal.fast_server?
+      %li.navBar__item= link_to "Tracking Domains", organization_server_track_domains_path(organization, @server), :class => ['navBar__link', active_nav == :track_domains ? 'is-active' : '']