Move all css files under `/static`

[Nutomic]

Some css files are loaded like /static/5b6438cf/styles/styles.css but others as /css/themes/browser.css. These should all be moved under /static with the hash, so they can be served as immutable.

This will let us simplify the cache-control logic:

lemmy-ui/src/server/middleware.ts

Line 47 in 0b3edaa

req.path.match(/\.(js|css|txt|manifest\.webmanifest)\/?$/) ||