Add static analysis job to find invalid YAML in changelogs

What does this MR do?

Finds invalid YAML in unreleased changelog entries, and fails the pipeline if that's the case.

Are there points in the code the reviewer needs to double check?

Don't think so.

Why was this MR needed?

Invalid YAML means that it won't be included in the compiled changelog.

@sindrepm noticed this in https://gitlab.com/gitlab-org/gitlab-ce/issues/23714#note_41582903.