set up flatpak ci

[Lunarequest]

Describe the feature or problem you’d like to solve

Currently we can't ensure that this continues building in a reliable manner.

Proposed solution

I would like to set up ci like obs and elementary using https://github.com/marketplace/actions/flatpak-builder it will require some work to setup automation to update sources but the end result should be ensuring we continue to work in flatpaks

[Lunarequest]

I also believe we should be able to publish successfully built versions to the beta branch of flathub allowing us to catch regressions in areas out side tests

[shiftkey]

Tagging this as help wanted to indicate that I'd love some help with adding this support to CI, so I can at least validate that I'm not affecting the downstream flatpak support. But I don't have the bandwidth to do this myself so I might close this out if it remains unresolved given https://github.com/flathub/io.github.shiftey.Desktop is there.