Add validation to ensure security MRs are approved
What does this MR do?
Includes a new validation into MergeRequestValidator
class to ensure
security merge requests have at least one approval.
Message displayed differs if the merge request is targeting 'master' or stable branches.
This validation added here mimics the same rule we currently have on Canonical and Security projects.
Related to gitlab-com/gl-infra/delivery#710 (closed)