Support flatkpak on linux

[holmesworcester]

Electron builder supports flatpak (though not flathub) and we've had some requests for this. Also, there's an issue with app icons on AppImage.

Whoever looks into this should try enabling all the linux packaging formats electron-builder can build, and see which ones work out of the box.

Then we can pick which ones we want to support and write e2e tests for them.

Note: we'll need some kind of update notification and a warning on desktop when people are using a deprecated version of Quiet and need to update, like we have on mobile, because these formats don't support self-updating automatically. See: #1613