Check migrations are reversible

doc/development/database_review.md

@@ -104,6 +104,8 @@ and details for a database reviewer:
     place them in `db/post_migrate` instead of `db/migrate`. Keep in mind
     that post migrations are executed post-deployment in production.
 - Check [timing guidelines for migrations](#timing-guidelines-for-migrations)
+- Check migrations are reversible and implement a `#down` method
+- Data migrations should be reversible, too or come with a description of how to reverse.
 - Query performance
   - Check for any obviously complex queries and queries the author specifically
     points out for review (if any)