LMS: Student Notes - Toggle Visibility (UX-1125)

This work styles the toggle notes on and off UI being added to the LMS. Included in this work is:

* partial abstraction of utility-based navigation styling/markup
* brief clean up of Sass partial organization (adding in modules)
* styling/rendering of toggle notes specific UI
* styling/rendering of toggle notes errors (both system and vendor errors)

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**Known Concerns**
* The calculator's styling and markup are particular to that element - the UI added here has been positioned around this element, but utility navigation should be more robust when supporting multiple items per course.
* This UI may not play well with the chat feature, which has not been reviewed.
* The vendor's error message is still displayed when the notes server cannot be reached
* The system message (for when there is an error in toggling) is limited in content and is currently not using a known system feedback pattern

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**TODOS**
* ~~Wire in the ``.has-error`` stateful class to ``.edx-notes-visibility`` element (display is controlled by this toggle)~~
* ~~Wire correctly the ``.is-active`` class on the ``.action-toggle-notes`` elements (currently added in HTML, but JS is not toggling class while maintaining state)~~

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**Screengrab - Notes Toggled Off**
![screen shot 2014-12-29 at 2 27 42 pm](https://cloud.githubusercontent.com/assets/163763/5572099/80697b54-8f67-11e4-9124-596c44e63898.png)

**Screengrab - Notes Toggled On**
![screen shot 2014-12-29 at 2 27 26 pm](https://cloud.githubusercontent.com/assets/163763/5572098/7b7c0eb8-8f67-11e4-9129-6a761a617b17.png)

**Screengrab - Notes Toggle with System Error**
![screen shot 2014-12-29 at 2 29 35 pm](https://cloud.githubusercontent.com/assets/163763/5572103/8c4a79b4-8f67-11e4-987c-027c07d91c7c.png)
Thanks!