Setting assignee who is not part of the organization fails

[mschoettle]

When migrating merge requests/issues where the assignee is not part of the organization (anymore), updating the issue/PR fails with a validation error.

I propose to retrieve the current members of the organization at some point once at the beginning (let me know where the best place would be). Then in convertAssignees an additional check can ensure that the assignee still exists as a member.

As an optional step could be to add the assignee to the issue body to have this still recorded.

An open issue is that if the owner is not an organization (but a regular user), is there an API with which members can be retrieved?