Create index for users.unconfirmed_email

What does this MR do?

Create index for users.unconfirmed_email

This speeds up the following query:

SELECT
    users.*
FROM
    users
WHERE
    users.unconfirmed_email = ?
ORDER BY
    users.id ASC
LIMIT 1

Context is https://gitlab.com/gitlab-org/gitlab-ce/issues/66958.

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.

Performance and Testing

• Plan without index (~10,800ms): https://explain.depesz.com/s/0VBw
• Plan with index (<1 ms): https://explain.depesz.com/s/7kQ9

More testing in https://gitlab.com/gitlab-org/gitlab-ce/snippets/1892156