DEV: allow nav items to override computed property values

Plugins need to override extra nav-items. Make sure these properties are
override-able.

See:
https://deprecations.emberjs.com/v3.x/#toc_computed-property-override

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

@@ -92,6 +92,45 @@ const ExtraNavItem = NavItem.extend({
       return (this._href = value);
     }
   }),
+  displayName: discourseComputed("displayName", {
+    get() {
+      if (this._displayName) {
+        return this._displayName;
+      }
+
+      return this.displayName;
+    },
+
+    set(key, value) {
+      return (this._displayName = value);
+    }
+  }),
+  filterMode: discourseComputed("filterMode", {
+    get() {
+      if (this._filterMode) {
+        return this._filterMode;
+      }
+
+      return this.filterMode;
+    },
+
+    set(key, value) {
+      return (this._filterMode = value);
+    }
+  }),
+  title: discourseComputed("title", {
+    get() {
+      if (this._title) {
+        return this._title;
+      }
+
+      return this.title;
+    },
+
+    set(key, value) {
+      return (this._title = value);
+    }
+  }),
 
   count: 0,