Enable CodeClimate SCSS Lint checks (#2886)

* add scss_lint to Gemfile

* add .scss-lint.yml

* fix warnings of scss-lint

* chmod -x styles/variables.scss

* Enable CodeClimate SCSS Lint checks

.codeclimate.yml

@@ -9,10 +9,13 @@ engines:
     enabled: true
   rubocop:
     enabled: true
+  scss-lint:
+    enabled: true
 ratings:
   paths:
   - "**.rb"
   - "**.js"
+  - "**.scss"
 exclude_paths:
 - spec/
 - vendor/asset