Official support

Ideally the Flatpak would be offically recomended on [mattermost.com](https://mattermost.com/download/#) when users are downloading it. This issue tracks the steps that might be needed for that to happen.

The original proposal to build a Flatpak is here: https://github.com/mattermost/desktop/issues/870. @SemaiCZE Has there been any upstream interest to officially support the Flatpak since then?

An intermediate step could be to have it listed as part of [GitHub releases](https://github.com/mattermost/desktop/releases/tag/v4.7.0) with an unofficial label (like the AppImage). Also it could be mentioned [in the install guide](https://docs.mattermost.com/install/desktop.html?src=dl#linux).

To do (in no particular order):

- [ ] Ensure feature parity between Flatpak and non-Flatpak releases.
- [ ] Explore if [electron-builder](https://github.com/electron-userland/electron-builder/pull/5711) could be used over extracting the tarball.
- I personally think using the tarball is pretty sensible, after all the Flatpak manifest is quite simple.
- [ ] See if the icon can be fixed without putting it in the repo.
- [ ] Ensure x-checker-data works properly.
- [ ] Ask upstream if they have any issues/concerns with officially supporting a Flatpak package.
- [ ] Investigate building the Flatpak as part of Mattermost's infrastructure.
- This means there'd be no GitHub repo like we have now. Instead it would look like [Mozilla's Firefox Flatpak](https://flathub.org/apps/details/org.mozilla.firefox) (notice there's no publisher link).
- See also: https://github.com/flathub/com.obsproject.Studio/issues/136