Adopt Flatpak as an official app distribution method

Whilst users can install the unofficial Mullvad Browser Flatpak from Flathub, officially maintaining it will give the following benefits:

1. Since it'll be official packaged, it'll also be updated alongside the other packages, so it'll always be up-to-date.
2. Users can know that it probably hasn't been manually tampered by a 3rd-party, since it's an official package.

[ruihildt]

Closing as duplicate of #6

It is planned, but we don't know yet when that will be done.