Support sha1 prefixes on the data server

[comntr]

This not only improves privacy, as the data server won't know the full hash of the url, but also makes the protocol less chatty. GitHub has this feature too: https://github.com/comntr/webext/commit/40632a24 leads to the full sha1. It allows to use as few as 4 hex digits for this particular commit, but returns a 404 if fewer digits are given. This seems to be the right way to go: if a client provides a too short prefix, the data server won't spend CPU time on walking the entire file system.