Flatpak package-specific UX for updater

We are aiming to officially support Flatpak for Mullvad Browser distribution on Linux. We need to either:

1. Add an update check+notification for Flatpak installs
2. Update the flathub package to support our existing in-place no-root required updater

Homebrew for macOS (for example) lets packagers specify their packages update themselves (using the auto_updates true flag) so perhaps Flatpak support a similar mechanism. If not, we will need a way to signal to users they can get an update from their package manager.