TNL-660: Toggle single note visibility.

**[Jira ticket - TNL-660] (https://openedx.atlassian.net/browse/TNL-660)**

**Acceptance Criteria:**
As a student, if I can already see my markup, I want to show or hide a note on the unit I'm viewing.
- When you hover over a markup, the note associated with that markup should be shown.
- When you click on a markup, that note should stay open until the note is closed.

**[Sandbox] (http://polesye.m.sandbox.edx.org)**

@explorerleslie FYI.
@tymofij @olmar Please review.
It's hidden at the moment. See line 280.