Use "Chosen" for metric picker, update cluster-view heatmap

Use the "Chosen" widget for the metric picker. I think it looks better than the combobox and provides more options for us to use.
Update the cluster-view heatmap to use a grid of cells instead of a single image. Each cell represents a server, and its color reflects the current load value. If you mouseover a cell the name of the server and current load value are displayed in a tooltip. Not as artistic as the "image" version, but more functional in my opinion.