Web interface for yle-dl

[tuomassalo]

This PR adds a simple web interface for yle-dl.

I tried to keep the current behaviour as-is and minimize the diff to existing files. The web interface is only activated if -e ENABLE_WEB_UI=1 is specified.

That said, the PR naturally adds complexity and moving parts to the process. I do understand if you'd rather keep the image minimal.

For now, the web UI offers no way to set yle-dl options.

6.webm

[taskinen]

Thanks for the PR, and for the video, it made the whole thing easy to
follow. The web UI does look useful, and the implementation seems well
thought out (opt-in, small diff to the existing files, relative URLs so it
proxies cleanly).

Still, I'm going to pass on merging it. I want to keep this project doing
one thing only: one-off downloads from the command line.
That's the whole point of the image.

None of that is a problem with your code. I'd honestly like to see this as
its own project. FROM taskinen/yle-dl gets you everything this image
has, you can release on your own schedule and make your own calls on
things like auth. Publish it somewhere and I'll link it from the README.