Developed a way to run the webapp on a remote or headless computer.

The webapp can now be started on a remote or headless computer, just
specify `--listen=address` to make it listen on an address other than
localhost. It'll print out the URL to open to access it.

This doesn't use HTTPS yet, because it'd need to generate a
certificate, and even if it generated a self-signed SSL certificate,
there'd be no easy way for the browser to verify it and avoid a MITM.

So `--listen` is a less secure but easier option; using ssh to forward
the webapp's port to the remote computer is more secure.

(I do have an idea for a way to do this entirely securely, making
the webapp set up the ssh port forwarding, which I have written down
in [[webapp]].. but it would be rather complicated to implement.)

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Made the webapp rescan for transfers after it's been used to change a
repository's group. Would have been easy, but I had to chase down a
cache invalidation bug.
