Add Flatpak builds

[alicerunsonfedora]

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Not everyone want to use the AppImage, Debian, or Snappy builds that we provide and would prefer to use Flatpak.

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We should add the ability to create Flatpak builds from our existing desktop codebase and upload packages to Flathub.

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• Keep using Snappy: From browsing Reddit alone, I notice that Snaps are a bit controversial and a lot of Linux users don't want to use it. Linux Mint has disabled snap support in their latest release, and other distros don't ship it.
• Use AppImage: AppImage is a bit buggy given some of the bug reports that have shown up here, in addition to sandboxing issues mentioned on Discord and Matrix.

Additional context
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• electron-builder doesn't support Flatpak building (yet).
• There is a workaround that we can try by reading the unpacked Linux build that electron-builder generates: [Feature Request] Publish to flathub WAClient/WALC#44 (comment).
• We do have Flathub forked at hyperspacedev/flathub for us to start the Flathub submission process.