Improved Plugins SCSS management

Smaller improvements and fixes to allow for easier theming through plugins. See https://meta.discourse.org/t/calling-the-sitecustomization-api-from-a-plugin/14845/8

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In Details this does:

 - 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)