Add substitutes for maintainers in Danger roulette

What does this MR do?

Related issue: !18711 (merged)

Due to the extreme shortage (internal) of DB maintainers, we've been assigning Reviewers and Traintainers to provide a second review in lieu of a Maintainer being available.

This adds a substitutes option to easily pass in as a backup list of people to choose when the original list of people doesn't have a matching result.

Open questions:

• What people group do we want to use in lieu of Maintainers? I've currently chosen Traintainers but this is being discussed here: !18711 (diffs, comment 231708356)
• Do we want to write some MARKDOWN when we use the substitute list?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Tests have been added but we'll need test before going live, probably.