Normalize and enforce single quotes in styles

- Replace stylelint property in package.json with .stylelintrc.js
- Add string-quotes stylelint rule
- Fix quotes

resources/assets/styles/common/_variables.scss

@@ -1,5 +1,5 @@
 /** Import Bootstrap functions */
-@import "~bootstrap/scss/functions";
+@import '~bootstrap/scss/functions';
 
 $theme-colors: (
   primary: #525ddc