Allow group owners to retry failed placeholder reassignment

Problem to solve

The import team has received requests for help where customers encountered failed placeholder reassignments and wished to retry or manually delete them. Currently, it's not possible to retry failed placeholder reassignments without Rails console access and the help of GitLab support.

Proposal

1. Add a retry reassignment button on the placeholder user table to allow group owners to retry failed placeholder reassignments.
2. Display reassignment failure message in the UI
3. Add internal event tracking to track when failed reassignment has been retried similar to existing placeholder reassignment events

Intended users

Group owners and administrators (users with permission to reassign placeholder users)

Feature Usage Metrics

Tracked using internal events consistent with existing placeholder reassignment events: #514952 (closed)

Does this feature require an audit event?

No