Added valid until text to invite email (#2599)

config/locales/en.yml

@@ -356,6 +356,7 @@ en:
         info: You have been invited to your own personal space by %{name}
         signup_info: To signup using your email, click the button below and follow the steps.
         signup_link: Sign Up
+        valid: "The invite is valid until: %{date}"
         signup:
           info: A user that was invited has signed up to use Greenlight.
           admins_link: Visit the Organization Page