[Discussion] Explore migrating the flatpak building process to Session's control

I'm part of the Session Desktop team and an avid user of the flatpak so I'm looking at getting the building and distribution process moved into our control. I'm following up on #81 and specifically to a comment left by @GeopJr:
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> > so i can cosign any releases which go out
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> I don't think it's possible right now. While there are plans to allow self-publishing to Flathub (AFAIK), it's not done yet.
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> The current workflow is very hands-off (Download latest deb from session-desktop, unpack it, install - Flathub bot checks for new releases, updates the urls & hashes and merges automatically - Builds are being done on Flathub's infrastructure) but due to the nature of session (privacy & security), I'd find it reasonable if you'd want to take over the whole process. I'd suggest going the f-droid route and host your own repo (I'm aware of [andyholmes/flatter](https://github.com/andyholmes/flatter) for static hosting on GH pages with signing support etc). If you create your own repo, let us know and we'll mark this as EOL.
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> Otherwise, as barthalion said, you can only "verify" that the flatpak is official through DNS (which would also require for this package to be marked as EOL for the new app ID, from `network.loki.Session` to `io.oxen.Session`)

I've done some digging and it seems the [suggested method on Flathub](https://docs.flathub.org/docs/for-app-authors/github-actions) is to use [this "Flatpak Builder" github action](https://github.com/marketplace/actions/flatpak-builder) to do the flatpak building and [deploying?](https://github.com/marketplace/actions/flatpak-builder#deployment-stage). I've not dealt with building a flatpak and publishing it to Flathub before and I'm a little confused by some of what I've read in the docs. **If we take over the building of the flatpak will the flatpak still be available through Flathub or only on a self-hosted page?**

Here are the few places in the docs that have led me to believe Session wouldn't be on Flathub anymore:

[The after submission section of the Flathub docs](https://docs.flathub.org/docs/for-app-authors/submission/#after-submission) implies the only way to get on FlatHub is via a PR which puts the manifest into the FlatHub org (which is what this `flathub/network.loki.Session` repo is)
> it will be marked as ready and merged into a new repository under the [Flathub GitHub organisation](https://github.com/flathub/) by the reviewers at a later time

[The publishing section of the flatpak docs](https://docs.flatpak.org/en/latest/publishing.html) also seems to separate being on Flathub and self-hosting.
> the most convenient and effective method is to use [Flathub](https://flathub.org/), which provides a large centralized repository of Flatpak applications. Alternatively, it is possible to host a repository yourself, or to distribute Flatpaks as single file bundles.

Ideally we'd like to have it available on Flathub as its a good point of distribution (that's what I use for a lot of my apps), but as @GeopJr mentioned `"due to the nature of session (privacy & security), [they'd] find it reasonable if [we'd] want to take over the whole process."`. So to start, I'd love to gather more information so we can make a more informed decision about what to do going forward. **Is there a pathway to us having control over the flatpak building process while still being listed on FlatHub?** Thanks.