add environment and per-branch review deploy

Note: this depends on some changes of the mel.vin services arriving late February or early March.

what will be done:

• https://git.mel.vin/help/ci/environments
• https://git.mel.vin/help/ci/review_apps/index.md
  • create some shared thing for this gitlab instance
  • could also add another subdomain

what will not be done:

• https://git.mel.vin/help/user/project/clusters/index
  • is a way to manage distributed deployment-ish stuff, requires per-project setup and server-side setup