Add indexes to fields commonly used in queries to build emails (#9551)

## Context

When I looked at the email queries earlier, it was clear could
potentially send nudges to candidates who couldn't do anything with them
(users whose accounts were blocked or locked for some reason). So we
added those filters.

This PR just adds indexes to those fields to help speed up queries. 

## Changes proposed in this pull request

This PR just adds indexes to those fields to help speed up queries.

db/migrate/20240711122121_add_indexes_to_email_related_filters.rb

@@ -0,0 +1,9 @@
+class AddIndexesToEmailRelatedFilters < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    add_index :candidates, :submission_blocked, algorithm: :concurrently
+    add_index :candidates, :account_locked, algorithm: :concurrently
+    add_index :candidates, :unsubscribed_from_emails, algorithm: :concurrently
+  end
+end

db/schema.rb

@@ -400,9 +400,12 @@
     t.boolean "submission_blocked", default: false, null: false
     t.boolean "account_locked", default: false, null: false
     t.string "account_recovery_status", default: "not_started", null: false
+    t.index ["account_locked"], name: "index_candidates_on_account_locked"
     t.index ["email_address"], name: "index_candidates_on_email_address", unique: true
     t.index ["fraud_match_id"], name: "index_candidates_on_fraud_match_id"
     t.index ["magic_link_token"], name: "index_candidates_on_magic_link_token", unique: true
+    t.index ["submission_blocked"], name: "index_candidates_on_submission_blocked"
+    t.index ["unsubscribed_from_emails"], name: "index_candidates_on_unsubscribed_from_emails"
   end
 
   create_table "chasers_sent", force: :cascade do |t|