Consider cache-control: public for /assets

[silverwind]

Feature Description

As requested on Discord, I think it makes sense to have the assets that have contenthash in their filename cacheable by intermediary proxies via cache-control: public to take off some load from the gitea process.

For files that do not have contenthash like serviceworker.js and more, I'm not sure. I think we may have to exclude them or (ideally) change them to have a contenthash.

Also I do think we can raise the default cache duration for contenthash assets significantly. They are essentially immutable.

[silverwind]

If serviceworker.js is the last holdout without contenthash, I would consider removing it. It serves no real purpose for Gitea, except being a unnecessary cache layer, while browser HTTP cache already works.