Consider using shared secrets for all the Review Apps

Currently, for each Review App, we create two secrets manually:

function create_application_secret() {
  local namespace="${KUBE_NAMESPACE}"
  local release="${CI_ENVIRONMENT_SLUG}"

  echoinfo "Creating the ${release}-gitlab-initial-root-password secret in the ${namespace} namespace..." true

  kubectl create secret generic --namespace "${namespace}" \
    "${release}-gitlab-initial-root-password" \
    --from-literal="password=${REVIEW_APPS_ROOT_PASSWORD}" \
    --dry-run -o json | kubectl apply -f -

  if [ -z "${REVIEW_APPS_EE_LICENSE}" ]; then echo "License not found" && return; fi

  echoinfo "Creating the ${release}-gitlab-license secret in the ${namespace} namespace..." true

  echo "${REVIEW_APPS_EE_LICENSE}" > /tmp/license.gitlab

  kubectl create secret generic --namespace "${namespace}" \
    "${release}-gitlab-license" \
    --from-file=license=/tmp/license.gitlab \
    --dry-run -o json | kubectl apply -f -
}

but there's no point in creating new secrets each time. We could just have two secrets that are reused by all the Review Apps.

We'd need to set gitlab.migrations.initialRootPassword.secret: https://gitlab.com/gitlab-org/charts/gitlab/blob/master/doc%2Finstallation%2Fcommand-line-options.md