Allow skipping of Merge Train syncs

What does this MR do?

This adds support for skipping Merge Train syncs by adding the "merge-train skip" tag to a commit message. This is necessary so we can reduce the number of redundant sync pipelines as part of our release process.

See gitlab-com/gl-infra/delivery#1258 (closed) for more information.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content