MDL-60621 js: Add ability to set a modal to be scrollable or not
With this change the modal can be configured to be scrollable or not.
If enabled to be 'scrollable', the modal's body will become scrollable
when the modal's height exceeds the browser's height. This is useful
in cases where the content in the modal is quite large and extends the
modal beyond the browser's height, which usually results in a bad UI.
If 'scrollable' is not explicitely configured, it will be set as 'true'
by default as in most cases it would be the expected behaviour.
