Keep using kilt for rendering

Directly using Crystal's ECR seems to be causing issues, so
don't use kemal's 'render' macro and patch 'content_for' to
have the same behavior as before Kemal v1.1.1

src/invidious.cr

@@ -16,7 +16,12 @@
 
 require "digest/md5"
 require "file_utils"
+
+# Require kemal, kilt, then our own overrides
 require "kemal"
+require "kilt"
+require "./ext/kemal_content_for.cr"
+
 require "athena-negotiation"
 require "openssl/hmac"
 require "option_parser"