Student Notes UI

This work is for [TNL-932] (https://openedx.atlassian.net/browse/TNL-932)

@polesye Ping me with any questions, please.

CC'ing @talbs @explorerleslie 
@polesye Okay, sorry this took so long. Here's what I've got: these are only unit-centric, no section or subsection. We'll treat each snippet as an individual note with individual comments and meta information to the right.