Convert CI to Github Actions

[peternewman]

Outstanding issues:
Issues :-(

Links

1. [L056] 404 https://ebu.io/project/cpa
2. [L080] 500 http://www.avalpa.com/the-key-values/15-free-software/33-opencaster
3. [L089] 302 https://developer.blender.org/diffusion/ → https://developer.blender.org/
4. [L185] https://libav.org/ Failed to open TCP connection to libav.org:443 (getaddrinfo: No address associated with hostname)
5. [L194] 404 https://bitbucket.org/fk/rgain
6. [L279] 404 https://youtube-eng.googleblog.com/

CPA you can probably advise but is it https://github.com/ebu/cpa-service-provider (if so where's the redirect? 😄 )
Looks like this is opencaster - https://github.com/aventuri/opencaster
I couldn't really tell, but I assume the blender link update would be fine to take.
libav is gone - https://en.wikipedia.org/wiki/Libav. We might be able to find an archive somewhere (but maybe it got merged into ffmpeg again?
rgain has a Python 3 fork - https://github.com/chaudum/rgain3
YouTube is #173

• Delete the old Travis file