[Errbit] Add secret_token
[Errbit] Add secret_token

This is used by Rails for things including signing sessions.

Once this (and the change in the deployment repo) is merged and deployed, we
can remove the equivalent code from alphagov-deployment.

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