Send environment deployment status to GitHub

Description

With GitHub service integration (https://gitlab.com/gitlab-org/gitlab-ee/issues/3836), we can send CI/CD pipeline status to GitHub pull requests, but GitHub also supports deployment status. We should send status updates when environments are deployed.

Proposal

Links / references

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml