Fix activity label contrast

[jakeob28]

When logged in as an instructor, currently portions of the activity labels currently have insufficient contrast per WCAG guidelines:

This PR fixes this by using italics instead of reduced opacity to indicate the activity identifier:

Let me know if you'd prefer a different method (other than italics) of indicating the assignment identifier. One alternative is just to use the parenthesis without any additional styling, which would look like:

or bolding the rest of the caption:

[bnmnetp]

So we made them very light because many instructors find them distracting and unsightly. I wonder if there is a way to satisfy WCAG guidelines while not drawing attention to them.

[pearcej]

What about parentheses and right justification, so it is out of the top-left primary viewing area?