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.
@hodgestar On second thought, I think `width` and `height` can be widget specific. Maybe it will be better to remove `height`, and use for widgets where it makes sense. It doesn't really make sense to use `height` for the two widgets we currently have.

I think its okay for us to have a `rowspan` and to not have a `height` because widgets that don't make use of a fixed height will make use of the dashboard's ability to accommodate dynamic heights.

Thoughts?