Reset notification email to default when used email got deleted

What does this MR do and why?

Reset notification email to default when used email got deleted.

Fixes #438600.

How to set up and validate locally

• Add a secondary email in http://gdk.test:3000/-/profile/emails.
• Use http://gdk.test:3000/rails/letter_opener/ to see and click the confirmation link.
• Now go to http://gdk.test:3000/-/profile/notifications and use that new email for any group.
• Trigger a notification to see that it's working. Tip: Enable Receive notifications about your own activity on the same page.
• Check LetterOpener again and confirm the new email is used.
• Now delete this Email again (in http://gdk.test:3000/-/profile/emails).
• Trigger another notification, and confirm that the primary email got used, not the deleted one.
• Also check that your earlier changes in http://gdk.test:3000/-/profile/notifications got reset.

database query plan

1. The where().each_batch() call results in SELECT queries like these:

SELECT
    "notification_settings"."id"
FROM
    "notification_settings"
WHERE
    "notification_settings"."user_id" = 1
    AND "notification_settings"."notification_email" = 'aaa@bbb.ccc'
ORDER BY
    "notification_settings"."id" ASC
LIMIT 1

👉 https://console.postgres.ai/gitlab/gitlab-production-main/sessions/30948/commands/96122

SELECT
    "notification_settings"."id"
FROM
    "notification_settings"
WHERE
    "notification_settings"."user_id" = 1
    AND "notification_settings"."notification_email" = 'aaa@bbb.ccc'
    AND "notification_settings"."id" >= 1
ORDER BY
    "notification_settings"."id" ASC
LIMIT 1 OFFSET 500

👉 https://postgres.ai/console/gitlab/gitlab-production-main/sessions/30948/commands/96123

2. And the update_all then updates each batch in bulk

UPDATE
    "notification_settings"
SET
    "notification_email" = NULL
WHERE
    "notification_settings"."user_id" = 1
    AND "notification_settings"."notification_email" = 'aaa@bbb.ccc'
    AND "notification_settings"."id" >= 1

👉 https://postgres.ai/console/gitlab/gitlab-production-main/sessions/30948/commands/96124

All 3 query plans recommend adding a specialised index (email+user), which is part of this MR now.