TNL-930: Turn off Student Notes when Harvard Annotation Tool is enabled.

Disables Student Notes if one of `textannotation`, `imageannotation`, `videoannotation` modules exist in the list of `Advanced Modules`.

There is still possibility to break `Student Notes` and `Harvard Annotation Tool`:
  - add one of `textannotation`, `imageannotation`, `videoannotation` to the `Advanced Modules` list;
  - add one of `textannotation`, `imageannotation`, `videoannotation` to the unit;
  - remove it form the `Advanced Modules` list => **You can enable Student Notes now. If you have both modules in the sequential, it breaks one of these modules.**

I think we should write a good note about this situation.

@tymofij , @olmar, @jmclaus, @srpearce  please review.

**sandbox:** http://studio.polesye.m.sandbox.edx.org/
> display name: Enable Student Notes in Course
> help: Enter true or false. If true, students can use the Student Notes feature.
> 
> I hesitate to use something like "Make Student Notes Visible" because that could raise questions about whether students can make notes even if this value is set to false, but can only see them if this is set to true. "Enable Student Notes in Course" indicates more clearly that a student can't perform any action with Notes at all unless this value is set to true.
> 
> If "Enable Student Notes in Course" is too long, I'd suggest changing the display name for edxnotes to "Enable Student Notes in Platform" (since fewer people will see this) and changing the display name for edxnotes_visibility to "Enable Student Notes".

This field is not user visible. So, I just removed display name, and updated help string so `Indicates current state of visibility of the Student Notes. This can be changed by a student to show/hide all notes in Platform.` For more information: https://openedx.atlassian.net/browse/TNL-661
@srpearce what do you think?