No form-required class on required checkbox labels

This is a followup to #2668298: Custom Search Module Conflict.
Labels for required checkboxes don't get the .form-required class.

In 2668298 #6 Mario Steinitz suggested a change but didn't provide a patch and a work around was used instead. 2668298-10 approved the change but later the issue was closed without comment.

I have rerolled his comment into a patch against 8.x-4.x. I checked out the repo as it was when he made that comment, applied and committed his change. I then checked out 8.x-4.x and merged with that commit. Some manual merging was required, and I did the best I could, but I don't understand the code involved very well, so it will likely need some help to prevent regressions.

The only other change I made was

-    <label{{ label.attributes }}>
+    <label{{ label.attributes.addClass(label_classes) }}>

so that the label classes assembled above would actually be applied.