Add support for manual actions

What does this MR do?

This implements a when: manual which allows a jobs to be marked as manual actions. Manual actions have to be explicitly executed by developers.

What are the relevant issue numbers?

This is to solve: https://gitlab.com/gitlab-org/gitlab-ce/issues/17010

Screenshots (if relevant)

In the issue.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added (This is planned for 8.11 as part of introducing Pipeline/Environments/Deployments API)
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

This is no longer based on: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5295. This is standalone change.