NanoHTTPD has issues with HTTP Keep-Alive, especially when other requests
are mixed in, like the /request-swap POST or perhaps the F-Droid HEAD to
fetch the ETag before the GET.
This disables gzip encoding and sets a Content Security Policy while I'm at
it. APKs, PNGs, and JARs are already compressed, so gzip would only ever
cause problems. And the index page is meant to be viewed by browsers, so
having a CSP will limit potential malicious swap activity.
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