This adds both redirect-to-signed-URL and proxy fileserving.
The proxy fileserving is somewhat limited: we don't support the Range header,
and it isn't easy to reuse the net/http ServeContent implementation because
that requires a SeekCloser. I think it might be possible to "bodge" a
SeekCloser on top of dynamically opening files, but it'll be a bit wonky and
will be slower than strictly necessary.
This allows setting the persistent storage location to save files into.
NOTE: defaults to mem:// for development purposes, which... is gonna be a bad
time.
This extracts the hash-named static assets into a directory. This is primarily
useful if you have multiple instances of fup running and you therefore need to
serve /static from a shared filesystem of some sort.
