TNL-932: Add styling to Notes page.

TNL-932
@tymofij, @jmclaus, @clrux please review.

**sandbox:** http://tymofij.m.sandbox.edx.org/courses/edX/DemoX/Demo_Course/edxnotes/
> Could we use something more descriptive than class="sr"?

We already have this class name in our platform. It was added by UI/UX team.