Update AbuseReportFinder to allow filtering by user

• Required for !110205 (merged)
• Implements some backend code for https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/157

What does this MR do and why?

This MR reuses AbuseReport.by_user_id scope in AbuseReportFinder to allow report filtering by user using a given username.

These filtering capabilities will be used for the updated Abuse Reports list introduced in !110205 (merged).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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