ECOM-6463: correct embargo URL

There's no such page as /embargo, it yields a 404.
/embargo/blocked-message/courseware/embargo/ is the correct URL.

playbooks/roles/nginx/templates/edx/app/nginx/sites-available/lms.j2

@@ -117,16 +117,17 @@ error_page {{ k }} {{ v }};
   {% include "common-settings.j2" %}
 
   {% if NGINX_EDXAPP_EMBARGO_CIDRS -%}
-  #only redirect to embargo when $embargo == true and $uri != /embargo
+  #only redirect to embargo when $embargo == true and $uri != $embargo_url
   #this is a hack to do multiple conditionals
+  set embargo_url "/embargo/blocked-message/courseware/embargo/"
   if ( $embargo ) {
     set $do_embargo "A";
   }
-  if ( $uri != "/embargo" ) {
+  if ( $uri != $embargo_url ) {
     set $do_embargo "${do_embargo}B";
   }
   if ( $do_embargo = "AB" ) {
-    return 302 /embargo;
+    return 302 $embargo_url;
   }
   {% endif -%}
 
@@ -174,7 +175,7 @@ error_page {{ k }} {{ v }};
   }
 
 {% if NGINX_EDXAPP_EMBARGO_CIDRS %}
-  location /embargo {
+  location $embargo_url {
     try_files $uri @proxy_to_lms_app;
   }
 {% endif %}