We use schema_migration_details to determine the age of a site in multiple migrations. This commit moves the logic into a dedicated `Migration::Helpers` module so that it doesn't need to be re-implemented every time.
FEATURE: Chat and Sidebar are now on by default
- Set the sidebar site setting to be enabled by default
- Set the chat site setting to be enabled by default
- Updated existing specs that assumed the original default
- Use a migration to keep old defaults for existing sites