Improved Plugins SCSS management

 - Moves the import of plugins for both mobile and desktop from common after discourse loading, allowing plugins to overwrite
 - Make desktop-option behave like the mobile-option: SCSS/CSS marked with that option will only be loaded for desktop from now on and ignored in mobile
 - Add variables-keyword, allowing plugins to ship and overwrite variables before they get imported by discourse (great for theming)
Merge pull request #2292 from ligthyear/sass-variables

Improved Plugins SCSS management