Allow dashboard widgets to have a dynamic height

At the moment, we require the height of each widget to be given explicitly. This doesn't work too well, since in many cases, a widget doesn't have a fixed height.
These need to be asymmetrical. Using `min-width` here will cause the widget to auto-expand to it's parent's width. We need to use `min-height` instead of height to avoid the contents overflowing the widget.

Only other alternative is to do this width and height setting at the end of the draw, but I think that's more work than it is worth right now.