Add support for "If-Modified-Since" and "Last-Modified"

[hugmouse]

Might be a fix for #222 , but might need some more testing. And maybe add support in a similar way for other static stuff in the project.

Tested in docker, seems to be working as expected.

[juev]

I did some research today on how this tag affects caching on the cloudflare side. alas, when I make requests, I always see the answer 200, never 304. And the file also starts to be given out of date even if checked with curl.

[bouncepaw]

Is development still active?

[hugmouse]

Nope. That was an experiment to try to fix issue 222. It can't fix the fact that you have to configure Cloudflare (or any other proxy for that matter) if you want this to work.