Allow mapping/reassignment from Placeholder-users to Service accounts and Group and Project bots via UI

On #537445 (closed), support will be added for mapping and reassigning from Placeholder-users to Service accounts using the API and CSV. This issue focuses on enabling reassignment through the UI. To facilitate this, we might need to add a new filter to the UserFinder class and the GraphQL Users.

Proposal

• (optional) add an additional (modal?) confirmation window as reassignment is automatic in this scenario, and cannot be cancelled
• Filter placeholder user reassignment drop-down list for service accounts that belong to the same top-level group