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.
@jmclaus I'm totally agree if we're talking about default events. But what about  this one `$(document).on('click.edxnotes:freeze', this.unfreeze.bind(this));`?
- Click on highlighted text -> bind `click.edxnotes:freeze`.
- Call destroy method. In this case, we must unbind `click.edxnotes:freeze`?
Am I right?