web: Load default Memory, CPU, TTY settings from image config

Closes #379
web: Refactor sliders into MemorySlider and CpuSlider objects

We want to use these sliders in multiple places. make them real
objects, albeit private to the docker code.
web: Read Docker container memory limit from CGroupMonitor

It could have changed since the container was launched, so don't
use the container config. We want to be able to react to changes
in the system appropriately.
web: Pull docker formatting of CPU and memory into util funcs

Once we start displaying this on the details page we need to
have this be cleaner.
web: Implement changing docker resources while cgroup running

Implement a rudimentary way of changing the memory limit and CPU
priority while the cgroup is running.