Add folder of initializers and move exception mailer config

This is to make it easier for us to override exception notifier settings
per-environment in the same way we do for rails.
Allow the ExceptionMailer subject prefix to be configured

This will let us override it in each environment so we have a clearer understanding
of where exceptions are coming from.
Correct path to aws_secrets.yml
Make aws secret paths consistent
Merge pull request #67 from alphagov/extract-initialisers

Extract initialisers