FIX: Use 1 column instead of 4 for permalink destination (#9260)

app/assets/javascripts/admin/models/permalink.js

@@ -1,4 +1,7 @@
 import { ajax } from "discourse/lib/ajax";
+import discourseComputed from "discourse-common/utils/decorators";
+import DiscourseURL from "discourse/lib/url";
+import Category from "discourse/models/category";
 import EmberObject from "@ember/object";
 
 const Permalink = EmberObject.extend({
@@ -13,6 +16,16 @@ const Permalink = EmberObject.extend({
     });
   },
 
+  @discourseComputed("category_id")
+  category: function(category_id) {
+    return Category.findById(category_id);
+  },
+
+  @discourseComputed("external_url")
+  linkIsExternal: function(external_url) {
+    return !DiscourseURL.isInternal(external_url);
+  },
+
   destroy: function() {
     return ajax("/admin/permalinks/" + this.id + ".json", {
       type: "DELETE"