Use icon instead of "Discussions" text, fix bugs

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

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;