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

Verification: converts all class-based Sass archetype-focused @extends to use Sass placeholder syntax

Verification: abstracting out specific color-based Sass variables to be *somewhat* friendly to theming

 Delay for possible race condition with fetching course email object.
 Use settings.SITE_NAME for host name to generate email footer url.

cleaned up variables styling to sync up closer to studio, incorporated fontawesome icons into discusison home with basic positioning

addition of font-awesome to LMS sass vendor assets, in additio nto light organization for applicaiton.scss to match studio sass breakdown.

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

- removed ::selection css from reset.scss
- removed variables from variables.scss
- removed variable calls from course/base/base.scss and base/base.scss

cleaned up variables styling to sync up closer to studio, incorporated fontawesome icons into discusison home with basic positioning

addition of font-awesome to LMS sass vendor assets, in additio nto light organization for applicaiton.scss to match studio sass breakdown.

lms - adds back in missing/previously added .sr screen reader-focused styling and references in place of the .visuallyhidden application

cleaned up variables styling to sync up closer to studio, incorporated fontawesome icons into discusison home with basic positioning

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