Disallow reopening of a locked merge request

Problem to solve

Similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/39665, a locked merge requests should not be allowed to get reopened.

Links / references

• Merge request that fixed https://gitlab.com/gitlab-org/gitlab-ce/issues/39665: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21299
• Example of a person keeping reopening a merge request: fdroid/fdroid-website!340 (closed)