Too many assignments on Gradebook page breaks layout

If you have more than 7-8 assignments in a group then they start to push over the edge of the content area into the block area.

This patch tries to rectify this by splitting the table into two parts, and then wrapping half of the table in a div, which can then have the following css applied to it:

div.scrollable {
  width: 600px;
  overflow-x: scroll;
}

More css styling is required to make it all look nice, but this is a start.