Absorb https://github.com/flathub/io.lbry.lbry-app and add to CI/CD pipelines

[ParaplegicRacehorse]

Community maintainted packages are great and all, but they are often not kept up to date. I have had issues with appimage auto-updates and I do not run a deb-based distribution. And the AUR package build has failed for me too often. I switched to the community maintained flatpak at flathub. (https://github.com/flathub/io.lbry.lbry-app)

Keeping up to date is much much easier with flatpaks than with appimages. Also, flatpak automagically gets added to application menu entries in desktop environments where appimage does not.

I feel like it would not be a great burden on the core desktop developers to pull the build files from the current flatpak maintainers and integrate into the existing CI/CD pipeline. After all, much of the work is already done.