Feedback app: configure secret key base as an envvar
Feedback app: configure secret key base as an envvar

This config is needed for Rails 4, which is what the app is being upgraded
to. This change brings the app closer to the 12factor way of configuration.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet