Provide a way to minimize the instructor/grading toolbar

Assignments in non-Canvas LMSes display a toolbar at the top when grading is enabled. We're planning to augment it with additional controls to initiate editing of an assignment (see https://github.com/hypothesis/lms/issues/4928). Assuming we do that, we'll also be displaying it, minus the grading elements, even when grading is not enabled. It takes up quite a bit of vertical space (~80px, so slightly under 1"), which leaves less space for the content when the instructor is not configuring or grading the assignment.

It would be nice to add some affordance for minimizing the toolbar, in order to give more space back to the content.

Slack thread: https://hypothes-is.slack.com/archives/C1M8NH76X/p1675342479143539