FIX: Check for permalinks before showing the 404 page

Limitations: the user profile "open external links in new tab setting" is
slightly broken for "External URL" permalinks.

Remove the copy from the admin permalinks page stating that this doesn't work.

test/javascripts/helpers/create-pretender.js.es6

@@ -267,12 +267,11 @@ export function applyDefaultHandlers(pretender) {
   pretender.put("/t/:id/recover", success);
   pretender.put("/t/:id/publish", success);
 
-  pretender.get("/404-body", () => {
-    return [
-      200,
-      { "Content-Type": "text/html" },
-      "<div class='page-not-found'>not found</div>"
-    ];
+  pretender.get("/permalink-check.json", () => {
+    return response({
+      found: false,
+      html: "<div class='page-not-found'>not found</div>"
+    });
   });
 
   pretender.delete("/draft.json", success);