Migrate assets to use Webpacker

app/views/favorites/create.js.erb

@@ -1,5 +1,5 @@
 <% if @error_msg %>
-  Danbooru.error("<%= @error_msg %>");
+  $(window).trigger("danbooru:error", "<%= @error_msg %>");
 <% else %>
   $("#add-to-favorites").hide();
   $("#add-fav-button").hide();
@@ -13,5 +13,5 @@
       $("#show-favlist-link").show();
     }
   <% end %>
-  Danbooru.notice("You have favorited this post");
+  $(window).trigger("danbooru:notice", "You have favorited this post");
 <% end %>

app/views/favorites/destroy.js.erb

@@ -10,4 +10,4 @@ $("#favcount-for-post-<%= @post.id %>").html(<%= @post.fav_count %>);
     $("#show-favlist-link, #hide-favlist-link, #favlist").hide();
   <% end %>
 <% end %>
-Danbooru.notice("You have unfavorited this post");
+$(window).trigger("danbooru:notice", "You have unfavorited this post");