Point AWS staging deployment buttons at the blue stack
Point AWS staging deployment buttons at the blue stack

- We have non-stack-specific DNS for `<appname>.staging.govuk.digital`,
  but when the redirect to the currently live stack (`blue`) happens,
  the path is not preserved.
- We're not going to be implementing a `green` stack any time soon, so
  hardcode `blue` to reduce confusion.

alphagov/release,alphagov/release,alphagov/release