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.
> HTML components --> annotatable component ?
> 
> Also, here: https://github.com/edx/edx-platform/pull/5896/files?diff=unified#diff-83c7abefacc56eb5a37caaaa66377c31R113 and in file in general please fix annotatatble-> annotatable

Done.