Tests and minor additions to mass mailer
Add MassMail model for MassMail actions

Updates the send_mail to use the MassMail model. This gives us validations,
automatic refilling of the form upon error, etc.
Add AdminController #send_mail spec
Merge pull request #186 from TeaWithStrangers/mailer-from-and-tests

Add 'From' Field to MassMailer View & Test E2E