Add notes to abuse report model

What does this MR do and why?

• Resolves sub-task 1 of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/168
• Creates a has-many relationship from the AbuseReport model to the Note model
• Creates Abuse::Reports::UserMention model & abuse_report_user_mentions table
• Skips namespace & project validation in Note model if the note is for an abuse report
• Adds AbuseReport as a noteable type

Database

• Output of db:migrate

main: == 20231003073437 CreateAbuseReportUserMentions: migrating ====================
main: -- create_table(:abuse_report_user_mentions)
main:    -> 0.0057s
main: == 20231003073437 CreateAbuseReportUserMentions: migrated (0.0136s) ===========

main: == 20231003073505 AddAbuseReportsForeignKeyToAbuseReportUserMentions: migrating 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE abuse_report_user_mentions ADD CONSTRAINT fk_088018ecd8 FOREIGN KEY (abuse_report_id) REFERENCES abuse_reports (id) ON DELETE CASCADE NOT VALID;")
main:    -> 0.0013s
main: -- execute("ALTER TABLE abuse_report_user_mentions VALIDATE CONSTRAINT fk_088018ecd8;")
main:    -> 0.0012s
main: == 20231003073505 AddAbuseReportsForeignKeyToAbuseReportUserMentions: migrated (0.0360s) 

main: == 20231003073526 AddNotesForeignKeyToAbuseReportUserMentions: migrating ======
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE abuse_report_user_mentions ADD CONSTRAINT fk_a4bd02b7df FOREIGN KEY (note_id) REFERENCES notes (id) ON DELETE CASCADE NOT VALID;")
main:    -> 0.0010s
main: -- execute("ALTER TABLE abuse_report_user_mentions VALIDATE CONSTRAINT fk_a4bd02b7df;")
main:    -> 0.0010s
main: == 20231003073526 AddNotesForeignKeyToAbuseReportUserMentions: migrated (0.0313s) 

• Output of db:rollback

main: == [advisory_lock_connection] object_id: 222340, pg_backend_pid: 109
main: == 20231003073526 AddNotesForeignKeyToAbuseReportUserMentions: reverting ======
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- remove_foreign_key(:abuse_report_user_mentions, {:column=>:note_id})
main:    -> 0.0209s
main: == 20231003073526 AddNotesForeignKeyToAbuseReportUserMentions: reverted (0.0398s) 

main: == 20231003073505 AddAbuseReportsForeignKeyToAbuseReportUserMentions: reverting 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- remove_foreign_key(:abuse_report_user_mentions, {:column=>:abuse_report_id})
main:    -> 0.0046s
main: == 20231003073505 AddAbuseReportsForeignKeyToAbuseReportUserMentions: reverted (0.0125s) 

main: == 20231003073437 CreateAbuseReportUserMentions: reverting ====================
main: -- drop_table(:abuse_report_user_mentions, {:if_exists=>true})
main:    -> 0.0016s
main: == 20231003073437 CreateAbuseReportUserMentions: reverted (0.0070s) ===========

How to set up and validate locally

1. Create an abuse report

> report = AbuseReport.create(user: User.last, reporter: User.first, message: "test")

2. Create a note and user-mention for the abuse report

> note = Note.create(noteable_type: "AbuseReport", noteable: report, note: "Test note", author: User.first)
> Abuse::Reports::UserMention.create(abuse_report: report, note: note, mentioned_users_ids: [1])

3. Test that the note and user-mention can be retrieved for the abuse report

> report.notes
> report.user_mentions

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.