Use Guice to inject an instance of the MailerAPI into controllers.

Previously we were using the Play plugin system, which was needlessly
different to everything else. Fixes #411.
Merge mailer changes (Play plugin to Guide) into helpdesk_integration

Conflicts:
	app/Global.scala
	test/helpers/TestConfiguration.scala
Remove unneeded plugin line
Merge branch 'master' into helpdesk_integration