replace lesshint with stylelint (#7305)

New CSS linter which is much more powerfull than the previous one.
Configuration is default but I had to remove a few rules that were
throwing too many or weird errors.

More importantly, the linter will exit with code 1 on errors so now our
build will fail if the CSS linter fails which should eliminate linter
errors being introduced without notice.

.stylelintrc

@@ -0,0 +1,11 @@
+extends: stylelint-config-standard
+
+rules:
+  block-closing-brace-empty-line-before: null
+  color-hex-length: null
+  comment-empty-line-before: null
+  declaration-empty-line-before: null
+  indentation: 4
+  no-descending-specificity: null
+  rule-empty-line-before: null
+  selector-pseudo-element-colon-notation: null