scss: introduce $form-title-style (#74757)

static/includes/wcs/_bulk.scss

@@ -7,6 +7,7 @@ $form-sidebar-gutter: 2% !default;
 // * page: full page width (i.e. sidebar + form content)
 // * form: form width (+ margin to be aligned on top of form)
 $form-titlebar-mode: page !default;
+$form-title-style: '%title' ! default; // class or sass placeholder selector or none
 $form-background: $body-background !default;
 
 // hide a bunch of elements
@@ -85,8 +86,11 @@ div#services > ul > li {
 }
 
 .form-content--title,
-div#tracking-code h3,
 div#rub_service h2 {
+	@extend #{$form-title-style} !optional;
+}
+
+div#tracking-code h3 {
 	@extend %title;
 }