Flatpak: Offer ref files as download

[danirabbit]

Problem

AppCenter apps can now be downloaded and used on other distributions, but it's not obvious how to do so

Proposal

Instead or maybe in addition to offering to download elementary OS to get these apps, offer the flatpak ref as a download

Prior Art

The workflow from Flathub seems to work out for this

[cassidyjames]

As a note, we'll need to separate the logic for Flatpak apps from legacy apps. In fact, it doesn't really make sense to offer to download elementary OS on any legacy apps if they're not available in OS 6. And we can change the wording around to talk about why the app was designed for/runs best on elementary OS (keeping a link to the site) while also offering a Flatpak download for other distros.