Use a more precise required/optional fields legend

Usability tests showed some users were struggling when seeing the word
"Required" as a legend (it's actually styled as a title), since this
isn't a common pattern when filling in forms. They were expecting
something like "Fields with * are required", and so the "Required" text
alone made them feel the sentence wasn't complete.

We could also add an "*" to required fields in addition to the
"Required" text. In this case, however, some users wondered what the
asterisk was about: "It can't mean 'required' because they've already
said these fields are required".

We'll probably have to revisit required and optional fields again. For
now, we're just doing a small improvement.

config/locales/en/general.yml

@@ -769,8 +769,8 @@ en:
         zero: "0 languages in use"
         one: "1 language in use"
         other: "%{count} languages in use"
-    optional: "Optional"
-    required: "Required"
+    optional: "Optional fields"
+    required: "Required fields"
   social:
     facebook: "%{org} Facebook"
     twitter: "%{org} Twitter"

config/locales/es/general.yml

@@ -769,8 +769,8 @@ es:
         zero: "0 idiomas en uso"
         one: "1 idioma en uso"
         other: "%{count} idiomas en uso"
-    optional: "Opcional"
-    required: "Obligatorio"
+    optional: "Campos opcionales"
+    required: "Campos obligatorios"
   social:
     facebook: "Facebook de %{org}"
     twitter: "Twitter de %{org}"