[Audit Events] Display user name instead of "Ghost"

Background

When a user is "deleted" without deleting their contributions, the user is actually migrated into the "Ghost User".

Problem to solve

The Audit Events page displays events associated with deleted users under the name ghost-user-1. These semi-anonymous entries are bad for auditing, because auditors need to know who made each change.

Example

Before user is deleted	After user is deleted

Proposal

AuditEvents capture both the User ID and the User name. If the User ID is associated with the Ghost User, display the user name instead. (This proposal courtesy of @tancnle.)