DEV: Explicitly allow NavItem customization (#17061)

Allow overriding NavItem's title and displayName. Fixes multiple instances of `computed-property.override` warnings in plugins.

app/assets/javascripts/discourse/app/models/nav-item.js

@@ -17,12 +17,27 @@ import { reads } from "@ember/object/computed";
 
 const NavItem = EmberObject.extend({
   @discourseComputed("name")
-  title(name) {
+  title: {
+    get(name) {
+      if (this._title) {
+        return this._title;
+      }
+
       return I18n.t("filters." + name.replace("/", ".") + ".help", {});
     },
 
+    set(value) {
+      this.set("_title", value);
+    },
+  },
+
   @discourseComputed("name", "count")
-  displayName(name, count) {
+  displayName: {
+    get(name, count) {
+      if (this._displayName) {
+        return this._displayName;
+      }
+
       count = count || 0;
 
       if (
@@ -40,6 +55,11 @@ const NavItem = EmberObject.extend({
       );
     },
 
+    set(value) {
+      this.set("_displayName", value);
+    },
+  },
+
   @discourseComputed("filterType", "category", "noSubcategories", "tagId")
   href(filterType, category, noSubcategories, tagId) {
     let customHref = null;