Auto-ban duplicate users when a user is banned

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/814

This MR implements the functionality when a user is banned to automatically ban other users that have the same detumbled email address. Email providers allow alternative addressing schemes that allow a user to modify their email address without changing the mailbox that the email is ultimately routed to. For example, user+123@gmail.com and user+456@gmail.com will both ultimately resolve to user@gmail.com.

Database changes

Change: Added Users.by_detumbled_emails scope

Single Detumbled Email

Raw SQL

SELECT "users".* FROM "users" 
INNER JOIN "emails" ON "emails"."user_id" = "users"."id" 
WHERE "emails"."detumbled_email" = 'user@gitlab.com'

Explain

https://postgres.ai/console/gitlab/gitlab-production-main/sessions/31992/commands/98961

 Nested Loop  (cost=1.00..7.03 rows=1 width=1487) (actual time=0.050..0.072 rows=3 loops=1)
   Buffers: shared hit=19
   I/O Timings: read=0.000 write=0.000
   ->  Index Scan using index_emails_on_detumbled_email on public.emails  (cost=0.56..3.58 rows=1 width=4) (actual time=0.025..0.028 rows=3 loops=1)
         Index Cond: (emails.detumbled_email = 'user@gitlab.com'::text)
         Buffers: shared hit=7
         I/O Timings: read=0.000 write=0.000
   ->  Index Scan using users_pkey on public.users  (cost=0.44..3.46 rows=1 width=1487) (actual time=0.012..0.012 rows=1 loops=3)
         Index Cond: (users.id = emails.user_id)
         Buffers: shared hit=12
         I/O Timings: read=0.000 write=0.000

Multiple Detumbled Emails

Raw SQL

SELECT "users".* FROM "users" 
INNER JOIN "emails" ON "emails"."user_id" = "users"."id" 
WHERE "emails"."detumbled_email" IN ('user@gitlab.com', 'user@example.com')

Explain

https://postgres.ai/console/gitlab/gitlab-production-main/sessions/31992/commands/98959a

 Nested Loop  (cost=1.00..14.06 rows=2 width=1487) (actual time=12.729..28.820 rows=3 loops=1)
   Buffers: shared hit=8 read=18
   I/O Timings: read=28.478 write=0.000
   ->  Index Scan using index_emails_on_detumbled_email on public.emails  (cost=0.56..7.15 rows=2 width=4) (actual time=8.302..18.350 rows=3 loops=1)
         Index Cond: (emails.detumbled_email = ANY ('{user@gitlab.com,user@example.com}'::text[]))
         Buffers: shared hit=4 read=10
         I/O Timings: read=18.182 write=0.000
   ->  Index Scan using users_pkey on public.users  (cost=0.44..3.46 rows=1 width=1487) (actual time=3.476..3.476 rows=1 loops=3)
         Index Cond: (users.id = emails.user_id)
         Buffers: shared hit=4 read=8
         I/O Timings: read=10.296 write=0.000

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Before	After

How to set up and validate locally

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

Example below:

1. In rails console enable the auto_ban_via_detumbled_email feature

   Feature.enable(:auto_ban_via_detumbled_email)

2. Create a user with a tumbled email (i.e. user+1@example.com)
3. Create another user with the same tumbled email (i.e. user+2@example.com).
4. Ban one of the new users. The other user should be automatically banned as well.
5. Retry the tests but add one of the users to a paid top-level namespace. When you ban the other user, the user in the paid namespace should be exempt from being auto-banned.