Flathub/Flatpak download option for Linux

A Flatpak option would make downloading and installing Morgen on Linux more accessible. Snap has been criticized due to not having an open source server and is not accessible by default on Fedora. Meanwhile, Flatpak is much more open in nature and supports management of the sandbox via Flatseal, giving users trust in Morgen's privacy. Also, many users look for apps via Flathub, which does not currently have Morgen.