web: Implement slider for choosing a container memory limit

This implements the memory row as described by the mockup here:

https://github.com/cockpit-project/cockpit-design/blob/f1d6e8c560d6230bad24e4ac11c8916dc75d9e52/docker/docker-run-hires.png

The only difference is that the total memory is not shown, as the
container is not actually being allocated a portion of the memory. This
is about setting a limit.