TNL-661: Toggle all markup visibility and ability to add new notes.

**Ticket:** [TNL-661](https://openedx.atlassian.net/browse/TNL-661)

**Acceptance Criteria:**
As a student, I want to show or hide all my notes when viewing courseware. 

- When viewing a unit in the courseware, show a toggle that shows/hides all markup.
- When markup is hidden, you also shouldn't have the option to create new markup. 
- This setting should persist from unit to unit (if you hide markup on one unit, the next unit you view should also have the markup hidden) and between sessions (if user logs out and logs back in, or closes window). 

**Sandbox:** http://jmclaus.m.sandbox.edx.org/

@explorerleslie Please review.
@tymofij , @olmar, @polesye please review.
Yeah, I'd agree that any customization can happen within our platform's Sass/CSS. Editing vendor code should be a very very last resort.