Fix box sizing for pseudo elements.

This PR tries to fix #6513.

In my testing, this has no adverse side-effects, but like the original ticket reporter suggests, we need to verify that there are no visual regressions. It would be good to test:

- metaboxes
- plugins
- all breakpoints
- browsers
- IE

edit-post/assets/stylesheets/main.scss

@@ -76,7 +76,9 @@ body.gutenberg-editor-page {
 }
 
 .gutenberg {
-	* {
+	*,
+	*:before,
+	*:after {
 		box-sizing: border-box;
 	}