Make FIDO U2F dongles accessible to the user session

This avoids having to install libu2f-host0 (which isn't discoverable at all) to
make those devices work.

Add this to debian/extra/rules/70-debian-uaccess.rules, as we might get similar
rules in the future. This was rejected upstream at
https://github.com/systemd/systemd/issues/102 because in some future this
should move to a proper kernel driver. Keep this downstream until that happens
and Chrome & friends move over to that.

LP: #1387908