Summary of changes:
* rename DOCKER_IMAGE to DEPLOYMENT_IMAGE
* docker image builds now run for any branch/tag
* include jobs for deploying and stopping the review app on kube
* review apps are deployed to the `fpco-untrusted` cluster
* review apps are available based the following URL scheme:
https://stackage-server-$CI_BUILD_REF_SLUG.review.fpco-untrusted.fpcomplete.com/
* master deploys to prod, and the previous staging env has been dropped
* import the kube specs we've used in CI/prod, update to use envvars
for APPROOT, DEPLOYMENT_APP, DEPLOYMENT_NAME, and DEPLOYMENT_IMAGE
