views: factor out FontAwesome icons.

Factor out FontAwesome icons into a set of helpers. This is so that it's
easier to keep track of which icons we're using and easier to change
icons globally.

app/components/popup_menu_component/popup_menu_component.html.erb

@@ -1,6 +1,6 @@
 <div class="popup-menu">
   <a class="popup-menu-button" href="javascript:void(0)">
-    <i class="fas fa-ellipsis-h"></i>
+    <%= ellipsis_icon %>
   </a>
 
   <ul class="popup-menu-content">