Add multi site support of pre and post migrators

The migrators should be classified as core related migrators and site related migrators. Because when a site is registered after the cms is bootstrapped, the core related migrator shouldn’t be run again. 

To be able to set site related migrator, new annotations could be defined. Also site related migrator could implement a new interface.

Site related migrators run when both the cms is being bootstrapped and a site is registered after bootstrapping.

Another thing that site related migrators need know the current site which is being registered. Because these migrators should be run once for each site configuration.