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/
@polesye not sure if we need to escape json and save escaped data in edx-notes service. I think it is better not to change data that will be stored in service.