🐛(react) disable button link

According to its prop, the Button component can be rendered as
a link but in this case, the prop `disabled` has no effect. So to fix
that if the Button receives a `href` prop and is disabled, we apply
a modifier class `.c__button--disabled`.

.changeset/fresh-dots-happen.md

@@ -0,0 +1,5 @@
+---
+"@openfun/cunningham-react": patch
+---
+
+Apply disable style to disabled button link