* API Replace existing Email and Mailer classes with SwiftMailer powered email system
* DOCS New Email docs
* Initial feedback from Damian
* Making Mailer an interface
* Templates relocated
* Round of feedback
* More robust approach to plain parts
* Revert changes to TestMailer
This small refactoring makes TestMailer better suited as a base class
for the behat-extension’s implementation, which means that we don’t
need to coordinate cross-module commits in dhensby’ SwiftMailer work.
See https://github.com/silverstripe/silverstripe-framework/pull/6466
