Merge branch 'manual-actions' into 'master'

Add support for manual CI 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

See merge request !5297