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();

js/forum/src/components/IndexPage.js

@@ -2,6 +2,7 @@ import { extend } from 'flarum/extend';
 import Page from 'flarum/components/Page';
 import ItemList from 'flarum/utils/ItemList';
 import listItems from 'flarum/helpers/listItems';
+import icon from 'flarum/helpers/icon';
 import DiscussionList from 'flarum/components/DiscussionList';
 import WelcomeHero from 'flarum/components/WelcomeHero';
 import DiscussionComposer from 'flarum/components/DiscussionComposer';
@@ -54,7 +55,7 @@ export default class IndexPage extends Page {
       app.cache.discussionList = new DiscussionList({params});
     }
 
-    app.history.push('index', app.translator.trans('core.forum.header.discussions_button'));
+    app.history.push('index', icon('bars'));
 
     this.bodyClass = 'App--index';
   }