Add an (optional) suffix to labels of optional fields

For now i use CSS because we are not sure whether we will keep it this
way or not and, until we finally decide, with CSS is the easiest to
remove later on.

web/static/numerus.css

@@ -295,6 +295,15 @@ input.width-2x {
 	top: 1rem;
 }
 
+[lang="en"] input:not([required]) + label::after {
+	content: " (optional)"
+}
+
+  [lang="ca"] input:not([required]) + label::after
+, [lang="es"] input:not([required]) + label::after {
+	content: " (optional)"
+}
+
 .input label, .input input:focus ~ label {
 	background-color: var(--numerus--background-color);
 	top: -.9rem;