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

shard.lock

@@ -20,6 +20,10 @@ shards:
     git: https://github.com/kemalcr/kemal.git
     version: 1.1.2
 
+  kilt:
+    git: https://github.com/jeromegn/kilt.git
+    version: 0.6.1
+
   lsquic:
     git: https://github.com/iv-org/lsquic.cr.git
     version: 2.18.1-2