Flatpak support and distribution via Flathub

[jzbz]

Resurrecting #2154, Flatpak is now the de facto "app store" for Linux and the main way software is installed in several major distros.

The main blocker for us has been that Electron builder doesn't have built in support for creating Flatpaks, the process for building them is documented here:

https://docs.flatpak.org/en/latest/electron.html

Once we have that we'd want to have distribution via Flathub and verify our "organization".

https://docs.flathub.org/docs/for-app-authors/submission/