Changed the way the user redirection urls are generated

Changed the way the user redirection urls are generated in the `contentCreateViewService` allowing users to overwrite the default values of the redirection attributes using plugins or another services.
it's weird to call the original implementation and just after maybe completely override its result... can't you just use https://github.com/ezsystems/PlatformUIBundle/pull/144#discussion_r22345226 ?