Add derailed_benchmarks gem.

app/assets/config/manifest.js

@@ -0,0 +1,8 @@
+// This is the config file for Sprockets. Danbooru doesn't use Sprockets, but
+// we still need this file anyway because the derailed_benchmarks gem doesn't
+// work without it. derailed_benchmarks does `require "rails/all"`, which loads
+// Sprockets, which fails if this config file isn't present.
+//
+// https://github.com/rails/sprockets-rails/issues/444
+// https://github.com/zombocom/derailed_benchmarks
+{}