Allow opting into legacy backend pages per route in the admin

Desired Behavior

Add configuration to allow selective use of old admin pages on a per route basis when the new admin is enabled. This will be configuration option that would be set in an initializer.

Current Behavior

You can select old or new but not choose a subset of pages to use the old admin on while having the new one enabled. (Old admin is still used when new admin doesn't implement the page currently)

Additional notes

Some stores have heavily customized old admin pages that will not easily port to the new admin. This will allow them to use the new admin while keeping their current heavily customized pages