Serve index.html and more with cache headers with expiration for appcache

[mattixpet]

• Just to be safe, e.g. appcache is a douche
• Actually, appcache should be replaced by Service Workers.

[judyfong]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching

"This may still not fix cache issues, but can't hurt" - Matthías