Add migration of User Personal settings when JIRA is integrated with an external User Repository (ie. Crowd)

Currently, the user properties are associated to the User IDs, not the user names.

If an external User repository is integrated with JIRA (ie. Crowd), on the first login, the user will be added to the external_entities table and it will receive a brand new ID. This ID will be different from the one being used in the user Personal Settings (propertyentry table), causing the users to loose the references to the data.

The propertyentry table also differentiates OSUser and External_entities entries using the ENTITY_NAME field. So, not only the ID will be different, but the ENTITY_NAME as well.

Since this use case is very common (Users were being stored in JIRA DB and then the customer decides to start using Crowd), there should be an automatic way to associate the old Personal Settings to the user in the new repository.