Add shutdown command; refresh containers list

The "stop" command on the main page kills containers without allowing services inside them to clean up after themselves. This patch adds a "shutdown" command which uses lxc-shutdown to stop containers. 

Since the shutdown may take some time, the containers list would still show the container as "running", until you manually refreshed the page. The containers list is now periodically refreshed (along with the host memory usage and cpu load). This has the added advantage that any changes that are e.g. performed from the command line will automatically become visible in the web panel.

The patch moves the template code for the containers list to a separate template, which is loaded from `/_refresh_containers` using `$.get`. Handlers for the destroy buttons are added.

(Created a new pull request from a separate branch)