UX: use icon for create topic btn on mobile (#31237)

Updates the create topic button to be icon only (mobile) due to screen
space restrictions. The icon is also updated to make it easier to
understand what the button does, even when there is no text.

app/assets/javascripts/discourse/app/components/create-topic-button.hbs

@@ -3,7 +3,7 @@
     <:button>
       <DButton
         @action={{this.action}}
-        @icon="plus"
+        @icon="far-pen-to-square"
         @disabled={{this.disabled}}
         @label={{this.label}}
         id="create-topic"

app/assets/javascripts/discourse/app/components/d-navigation.js

@@ -20,7 +20,9 @@ export default class DNavigation extends Component {
 
   @setting("fixed_category_positions") fixedCategoryPositions;
 
-  createTopicLabel = "topic.create";
+  get createTopicLabel() {
+    return this.site.desktopView ? "topic.create" : "";
+  }
 
   @dependentKeyCompat
   get filterType() {