remove unnecessary parentheses

change css properties for mixins bi-aap in login/register page

change css properties for mixins bi-app in about course page

change css properties for mixins bi-app in the dashboard

change css properties for mixins bi-app in help template

change css properties for mixins bi-app in the home page

change css properties for mixins bi-app in jobs, press and media pages

remove the extra spaces

- Icon images can be uploaded from the django admin
- Test coverage improved

- Icon images can be uploaded from the django admin
- Test coverage improved

ECOM-1002 fixed issue of grey background for logistration page. Also fixed focus issue when toggling between forms - as raised by @cptvitamin

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.

Reuse the style from login and register views.
Remove all the specific style.

- 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

edx.org - replaces login/register semantically verbose fieldset and legend elements with simpler markup

LMS/Accounts: Converts many class-based Sass archetype-focused @extends to use Sass placeholder syntax

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

The official border-radius mixins were deprecated and removed in Bourbon 3.0.
https://github.com/thoughtbot/bourbon/pull/95

theming - revised variables and link/button styling to account for edX's registration and login constrast/styling changes