Extract search indexing to service subscriber

Callbacks still there for archiving, unpublishing and deleting until
those actions are pulled out to service objects.
Move supporting page indexing to service object

Once again, there is still observer code for unpublishing and
archiving that needs pulling out later.
Move related edition indexing to service object

The ReindexRelatedEditions class is still in the observer for now and
will need to be moved elsewhere once we have the unpublish service
object.