Peertube video support

Hi,

Peertube is an opensource decentralised video platform that is slowly growing (and instances where created for French teacher).

It is already possible to use Peertube video in the interactive video activity, but you have to use the download url of the video and not the url of the peertube video page so teacher might thinks they can't use peertube video.

I started working on a way to handle it better and was able to get something functionnal but not sure if it's the best way to do it as it's a little dirty (by editing a variable that should be read only).

I don't know what h5p devs thinks of this solution.

Here the github issue with more information : https://github.com/h5p/h5p-video/issues/40