Allow reassignment of placeholder when source_hostname do not have a schema

On !167690 (merged), the source_hostname was updated to store the schema (HTTP or HTTPS) and port (80, 443, etc). Besides, a validation was introduced to check if the source_hostname has the schema and port.

Problem

Import source users created before !167690 (merged) still use the old format, and when group owners try to perform the reassignment they get the error: "Source hostname must contain scheme and host, and not path" error."

Proposed solution

Do not run the source_hostname validation for already created import source users.