Reversible migration helper

What does this MR do?

This MR introduces a reversible_migration migration helper.

Use the reversible_migration helper to test migrations that are reversible (ones that have either a change or both an up and down that perform changes). This can test that the state of the application and its data after the migration is reversed is the same as it was before the migration ran in the first place. The helper:

1. Runs the before expectations before the up migration
2. Migrates up
3. Runs the after expectations
4. Migrates down
5. Runs the before expectations a second time

Example:

reversible_migration do |migration|
  migration.before -> {
    # ... pre-migration expectations
  }

  migration.after -> {
    # ... post-migration expectations
  }
end

Does this MR meet the acceptance criteria?

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

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers