Support other users as owners of non-System containers

[openaiken]

When I have administrative permissions enabled on my cockpit session, I should be able to select another user than the one I am actively logged in as, as the owner of a new container. This should be a dropdown or autocompleted text entry box of existing local users on the host system. The selected user should apply to the ownership of the directories, files, and container processes.

[jelly]

This is a duplicate of #716 more or less.

The way cockpit-podman works won't implement it this way as the logged in user is the user connecting to the podman socket. We don't know what users might have a container running so we would have to try to detect a podman socket for all available users which doesn't scale.