Clarify and reformat some comments; rename a few functions to clarify those.

Convert tabs to spaces.

Fix inaccurate comment

Fixed jshint issues

Adjust how user-select is applied to the gradebook view. Add i18n for a string.

Allow selection when not dragging.

This is a squash of 38 commits ending with
5b080f979d692804452400ac5bed9b17c50b001e

SASS compilation variable conventions update, focusing on basic colors, baseline variable usage for padding and margins, ui-depth extends, along with other small fixes. The intent with this is to not have any user-facing impact, and instead move us closer toward having files that use our newer sass conventions.

- Doubled up :hover and :focus styling to improve a11y
- Increase contrast of certain UI elements for improved a11y
- Added some image alt text for a11y
- Changed video caption styling to blue and made them underline on hover and added a skip link before video for screenreaders.

Fixes Bugs:
- LMS-1336

Bourbon's box-shadow mixin has been deprecated, and they recommend to use the
official spec.

--HG--
branch : bridger-grades

--HG--
branch : rl-gradebook