Part 2 - Prevent emails from being sent without subject and/or body

Part 1 of this issue is THREESCALE-8009.

When sending emails from the bulk dropdown menu (applications, accounts or subscriptions pages) OR the account show page (clicking on send message) all fields can be left empty and the email will be sent regardless.

We should either:

• Disable the sent button until both fields are filled in or
• return an error message when submitted with empty fields