Official Flatpak support

Flathub: https://flathub.org/apps/net.mullvad.MullvadBrowser github: https://github.com/flathub/net.mullvad.MullvadBrowser

This is a community effort and a pretty popular way to install Mullvad Browser it seems (~25k downloads)

• Identify any differences in browser fingerprint between Flatpak package and ordinary tarball #203
• Flatpak package-specific UX for updater #204
• Flatpak build target in tor-browser-build #205
• Signing script updates to support publishing/signing Flatpaks #206
• Add correct Appstream metadata #245

Longer term we could see about reusing this work for tor-browser.