backport from eoan

We currently get youtube-dl from the WebUpd8 repo, which has two problems

• WebUpd8 does not package youtube-dl for Bionic, which could be a problem for Trisquel 9
• Although the version of youtube-dl in WebUpd8 is newer than that in Xenial and Bionic, it is older than the oldest version that works

Since YouTube occasionally breaks youtube-dl, and releases are so fast that bug fixes don't seem to get reliably backported by Debian or Ubuntu, maybe this is a package that should be rolling like Abrowser. I can't find an up-to-date PPA, but we can use Ubuntu testing. However, the version in Eoan wants debhelper 12, so maybe this won't work.

This merge request is for Flidas because Flidas users are currently affected but Etiona has the same problem, because the version of youtube-dl in Bionic is also broken.