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.

public/less/_form.less

@@ -211,9 +211,7 @@
             margin-left: 25px;
         }
     }
-}
 
-.new.webhook {
     .events.fields {
         .column {
             padding-left: 40px;