Gives us a single call site to trigger the destruction of vehicle
journeys, journey patterns, and routes without content.

This was previously done directly in the `#clean` method, but since it's
not needed during referential duplication (only during merges), we don't
want to enable it by default. Thus now to activate the same old
functionality, you would create a `CleanUp` like this:

    CleanUp.new(methods: [:destroy_empty])

Refs #6854

This method is coupled with a new virtual attribute that can be used in
the initializer like:

    CleanUp.new(methods: [:destroy_routes])

The method will run all methods specified in the `:methods` list. The
plan is to replace the calls to `destroy_routes` etc. with a call to
this method. The result will be a more configurable clean-up process,
allowing users to selectively choose what to clean up by declaring what
methods in the `CleanUp` model to call.

Refs #6854

Looks like I temporarily forgot that this method takes an ID.

Refs #6854

Ensure that `JourneyPattern`s and `VehicleJourney`s associated with
orphaned routes get deleted in cascade.

Refs #6854

This builds on `#destroy_vehicle_journeys_outside_referential` (which
will soon be removed in favour of this new method).

It destroys orphaned routes in a referential.

Refs #6854