Add cache headers to static resources

[earthiverse]

• Pretty format the YAML file
• Add Cache-Control headers to images, css, and sounds (things that are unlikely to change between versions)

[thmsndk]

How do we force the cache to reset then, if they in fact change between versions? We can't instruct all players to open dev tools and disable cache :P

Some sort of cache busting mechanism is important.

[earthiverse]

A lot of the URLs in this game have ?v=### appended to them. This breaks the cache when you increment the number.

[FreezePhoenix]

Earthiverse is right - all that has to be done is change the URL parameters used to access it, and voila - your browser is convinced it is a new file.

[FreezePhoenix]

I would also like to mention that although they were not marked with cache before, many browsers are intelligent enough to still cache them.

[Telokis]

Hey, thanks for this PR!
Where there is caching, I would like to see etag as well. Does this automaticall handle it or does it require additional configuration?
Once etag is enabled, we can increase the caching duration even more.

[earthiverse]

So, I took a look at this again, and I discovered a few things

1. expiration: 1d is the correct way to add expiration according to https://cloud.google.com/appengine/docs/standard/reference/app-yaml#handlers_expiration
2. The default cache timeout is 10 minutes according to https://cloud.google.com/appengine/docs/standard/how-requests-are-handled?tab=python#static_cache_expiration
3. It doesn't look like it's being applied. It might be because we're using a dev version of the app engine. I'm not sure how to mimic "production".
4. ETags are automatically added. Using thmsn's server, it returns 304s (Not modified) for all the resources.

[Telokis]

@earthiverse Ok, thanks for clarifying. I can't access any of the links you've provided, they all return "Service unavailable", weird.

In any case, I think we can proceed with the setup you've specified and then we'll see how it behaves and make changes accordingly.

I see you've marked it as draft, I assume you want to test or figure some things out before merging so feel free to tag me when I can review it again and you consider it ready!