Include instance in suspension reminder subject.

To avoid confusion, we want to let people know whether we're sending
them a reminder for production Signon, for a testing environment, or
even a completely separate instance. We can configure this through an
initialiser that we overwrite on deploy.
Include the remaining time in the subject.

Also add tests for the equivalent content in the email body parts.

Testing the content of rendered emails is probably stretching the
definition of a unit test, but the official Rails testing documentation
<http://guides.rubyonrails.org/testing.html#testing-your-mailers>
does it this way.
Update the reminder emails with new bodies.

These now make reference to the environment in the body, to give people
an extra chance to notice when it's for a non-production account.
Rename initialiser to match the naming convention.
Merge pull request #171 from alphagov/environment-in-emails

Include the environment in non-production reminders