This PR adds support for inv_sig_helper, which offloads the player fetching,
function extraction and signature parsing, which in turn allows to use the
web client to watch videos.
When the new config option "signature_server" is not set, the logic for the
external signature server is not enabled and invidious behaves like before.
This PR also updates the crystal overrides because the stdlib changed quite
a while ago (See issue 11049 at crystal-lang/crystal) and those were required
to properly use TCP/unix sockets.
Closes issue 4649
2d7869b48b