LPS-34567 Site template page import causes ConstraintViolation

Hey Brian,

These are the required changes in master. I've also sent you the backport for ee-6.1.x brianchandotcom/liferay-portal-ee/pull/1472. The main differences are:

* Part of the affected logic in LayoutImporter has been moved to LayoutStagedModelDataHandler.
* Using LayoutFriendlyURLUtil to check conflicts in any existing translations. Notice that the same layout might have the same friendlyURL for different translations, but the same friendlyURL cannot be used in other layout for any of its languages. 
* merge_alert.jsp already existed in 6.2.x so code has been combined with the existing one. 

Thanks

@lipusz 
@akos