If you configure "E-mail Notification" as an action for a promotion it does not
actually occur when the promotion occurs.

Cause: Mailer extends Publisher extends BuildStepCompatibilityLayer without
overriding BuildStepCompatibilityLayer's version of perform(AbstractBuild...)
therefore perform() does nothing except return true.

Fix: Override the new perform signature in Mailer.

Getting this to work involved removing a whole swag of pointless generics in
MailSender which doesn't actually care about the specific type of AbstractBuild
or AbstractProject so wildcards can be used, simplifying things quite a lot.