Prepend custom assets to CONSUL assets

Rails 5 changed the initialization order, and now our initializers were
running before the `append_assets_path` initializer for each engine,
which prepended application assets to the custom assets we prepended in
the initializer.

Moving the code to the `config.after_initialize` code didn't work
either, since the paths added there were ignored by the application.

Adding another initializer to the Rails Engine is a hack, but solves the
problem.

config/application.rb

@@ -71,4 +71,14 @@ module Consul
   end
 end
 
+class Rails::Engine
+  initializer :prepend_custom_assets_path, group: :all do |app|
+    if self.class.name == "Consul::Application"
+      %w[images fonts javascripts].each do |asset|
+        app.config.assets.paths.unshift(Rails.root.join("app", "assets", asset, "custom").to_s)
+      end
+    end
+  end
+end
+
 require "./config/application_custom.rb"