Flatpak for application containment

Epic for introducing flatpak.

As discussed for quite a while and also presented at Birmingham AMM

Tasks and goals needs some more work but to start it off:

• Include layer meta-flatpak
• Use bubblewrap from the System UI/application manager, to launch apps in namespaces (file, network etc) and other constraints. Introducing bubblewrap can be done by itself.
• Discovery task to write up a plan for how flatpak runtimes might be used to partition parts of the system that only one or a few applications depend on.