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)

lib/plugin/instance.rb

@@ -222,12 +222,14 @@ class Plugin::Instance
           DiscoursePluginRegistry.javascripts << asset
         end
       elsif asset =~ /\.css$|\.scss$/
-
-        unless opts == :mobile
-          DiscoursePluginRegistry.stylesheets << asset
-        end
-        unless opts == :desktop
+        if opts == :mobile
           DiscoursePluginRegistry.mobile_stylesheets << asset
+        elsif opts == :desktop
+          DiscoursePluginRegistry.desktop_stylesheets << asset
+        elsif opts == :variables
+          DiscoursePluginRegistry.sass_variables << asset
+        else
+          DiscoursePluginRegistry.stylesheets << asset
         end
 
       elsif asset =~ /\.js\.handlebars$/