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/views/layouts/default.html.erb

@@ -53,8 +53,8 @@
     <%= link_to Danbooru.config.app_name, root_path, id: "app-name-header", class: "heading" %>
 
     <div id="maintoggle" class="mobile-only">
-      <a href="#"><i id="maintoggle-on" class="fas fa-bars"></i></a>
-      <a href="#"><i id="maintoggle-off" class="fas fa-times" style="display: none;"></i></a>
+      <a href="#"><%= menu_icon(id: "maintoggle-on") %></a>
+      <a href="#"><%= close_icon(id: "maintoggle-off", style: "display: none;") %></a>
     </div>
 
     <nav id="nav">