Update source user mapping to support unstrusted source hostnames

In offline transfer, the destination instance does not make any API requests directly to the source instance. Instead, source user mapping will rely on the importing user to supply a source hostname when importing using offline transfer, and there's no way for GitLab to verify the legitimacy of the hostname.

The notifications sent to users who have been reassigned to contributions created from an offline transfer must be updated to reflect that these hosts aren't trusted. It must be clear to the reassigned user that the source host was supplied by the user and the import was created from a file-based import process.