FIX: check permalinks for deleted topics

- allow to specify 410 vs 404 in Discourse::NotFound exception
- remove unused `permalink_redirect_or_not_found` which
- handle JS side links to topics via Discourse-Xhr-Redirect mechanism

app/assets/javascripts/discourse/lib/ajax.js.es6

@@ -29,6 +29,17 @@ export function handleLogoff(xhr) {
   }
 }
 
+function handleRedirect(data) {
+  if (
+    data &&
+    data.getResponseHeader &&
+    data.getResponseHeader("Discourse-Xhr-Redirect")
+  ) {
+    window.location.replace(data.responseText);
+    window.location.reload();
+  }
+}
+
 /**
   Our own $.ajax method. Makes sure the .then method executes in an Ember runloop
   for performance reasons. Also automatically adjusts the URL to support installs
@@ -76,6 +87,7 @@ export function ajax() {
     }
 
     args.success = (data, textStatus, xhr) => {
+      handleRedirect(data);
       handleLogoff(xhr);
 
       Ember.run(() => {