Use icon instead of "Discussions" text, fix bugs

- Fix admin page crash
- Only show invisible pin button on desktop; hide it completely on ≤ tablet

js/admin/src/initializers/boot.js

@@ -31,6 +31,7 @@ export default function boot(app) {
   app.alerts = m.mount(document.getElementById('alerts'), AlertManager.component());
   app.history = {
     canGoBack: () => true,
+    getPrevious: () => {},
     backUrl: () => app.forum.attribute('baseUrl'),
     back: function() {
       window.location = this.backUrl();