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';
   }

less/forum/DiscussionPage.less

@@ -70,7 +70,6 @@
 @media @tablet-up {
   .DiscussionPage-list {
     left: -@pane-width - 6px;
-    width: 100%;
     position: absolute;
     z-index: @zindex-pane;
     overflow: auto;

less/lib/Navigation.less

@@ -5,8 +5,14 @@
   max-width: 150px;
   overflow: hidden;
   text-overflow: ellipsis;
+  padding-left: 8px;
+
+  .icon {
+    font-size: 14px;
+  }
 }
 .Navigation-pin {
+  display: none;
   opacity: 0;
   margin-left: -5px !important;
   border-radius: 0 @border-radius @border-radius 0;
@@ -18,6 +24,9 @@
 }
 
 @media @desktop-hd {
+  .Navigation-pin {
+    display: block;
+  }
   .hasPane.panePinned, .hasPane.paneShowing {
     .Navigation-back {
       border-radius: @border-radius 0 0 @border-radius !important;