Make an official Flatpak on Flathub

(#948) Feature Shipped packaging

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Comment by @spectrapulse
RexSystem 1 vote originally by @spectrapulse on GitHub
> With that said, this doesn't mean there shouldn't be an unofficially supported flatpak, I just don't want to end up with a situation where a flatpak is the only option which does happen. Why wouldn't we want an universal format that works on most* linux systems and is easy for the maintainer?
Because of the very reasons I've mentioned in the very reply you're quoting