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

app/javascript/styles/variables.scss

@@ -6,3 +6,5 @@ $color5: #ffffff !default; // white
 $color6: #df405a !default; // error red
 $color7: #79bd9a !default; // succ green
 $color8: #000000 !default; // black
+$color9: #ff5050 !default; // red
+$color10: #ca8f04 !default; // dark goldenrod