[Gradebook] Improve gradebook horizontal scroll bar

[caerfalias]

Problem statement:

The only way to scroll horizontally through my gradebook is to first scroll down, then there is a TINY bar I have to click and hold quickly before it disappears again. It is ridiculous to have to fight for that scroll bar. I wanted to add a screenshot, but I can't see where to do it here.

Proposed solution:

The gradebook horizontal scroll bar should always be visible, and preferably at the top of the page.

User role(s):

instructor