[Project service] Platform.sh Continuous Deployment

Now we have the CI runners doing their job, the next step is to add an extra stage to deploy to a QA environment.

We are using Platform.sh to automatically build containers based on the related Git branch and another simple config file.

We would like to be able to integrate this better into Gitlab. There is some information that can be integrated:

add Platform.sh as new Project service, allow to connect both branches
keep the Platform.sh branch in sync on every push / branch creation
allow Pipelines to startup an environment at Platform.sh
show the status and URL of the Platform.sh environment with the issue and MR
allow to shutdown an environment at Platform.sh when a MR is merged (read: branch is removed)
Gitlab webterminal integration

Remaining question: after the QA stage has been approved, the staging step in the pipeline should be triggered. How will that work?