Add database index on emails to detumbled_email column

What does this MR do and why?

Add database index on emails to detumbled_email column. This was missed in the original migration that added the column.

MR acceptance checklist

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Screenshots or screen recordings

Up Migration

▶ bundle exec rake db:migrate
main: == [advisory_lock_connection] object_id: 128220, pg_backend_pid: 558876
main: == 20240809201229 AddIndexOnEmailsToDetumbledEmail: migrating =================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0033s
main: -- index_exists?(:emails, :detumbled_email, {:name=>"index_emails_on_detumbled_email", :algorithm=>:concurrently})
main:    -> 0.0038s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:emails, :detumbled_email, {:name=>"index_emails_on_detumbled_email", :algorithm=>:concurrently})
main:    -> 0.0015s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20240809201229 AddIndexOnEmailsToDetumbledEmail: migrated (0.0206s) ========

main: == [advisory_lock_connection] object_id: 128220, pg_backend_pid: 558876

Down Migration

▶ bundle exec rake db:migrate:down:main VERSION=20240809201229
main: == [advisory_lock_connection] object_id: 127900, pg_backend_pid: 558021
main: == 20240809201229 AddIndexOnEmailsToDetumbledEmail: reverting =================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0027s
main: -- indexes(:emails)
main:    -> 0.0064s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:emails, {:algorithm=>:concurrently, :name=>"index_emails_on_detumbled_email"})
main:    -> 0.0012s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240809201229 AddIndexOnEmailsToDetumbledEmail: reverted (0.0250s) ========

main: == [advisory_lock_connection] object_id: 127900, pg_backend_pid: 558021

How to set up and validate locally

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