Create index for users.unconfirmed_email

This speeds up the following query:

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

Presumably, this is a query coming from Devise.

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

changelogs/unreleased/ab-unconfirmed-email-index.yml

+---
+title: Create index for users.unconfirmed_email
+merge_request: 32664
+author:
+type: performance

db/migrate/20190904173203_add_index_on_users_unconfirmed_email.rb

+# frozen_string_literal: true
+
+class AddIndexOnUsersUnconfirmedEmail < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :users, :unconfirmed_email, where: 'unconfirmed_email IS NOT NULL'
+  end
+
+  def down
+    remove_concurrent_index :users, :unconfirmed_email, where: 'unconfirmed_email IS NOT NULL'
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_09_02_160015) do
+ActiveRecord::Schema.define(version: 2019_09_04_173203) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -3562,6 +3562,7 @@ ActiveRecord::Schema.define(version: 2019_09_02_160015) do
     t.index ["state"], name: "index_users_on_state"
     t.index ["state"], name: "index_users_on_state_and_internal", where: "(ghost IS NOT TRUE)"
     t.index ["state"], name: "index_users_on_state_and_internal_ee", where: "((ghost IS NOT TRUE) AND (bot_type IS NULL))"
+    t.index ["unconfirmed_email"], name: "index_users_on_unconfirmed_email", where: "(unconfirmed_email IS NOT NULL)"
     t.index ["username"], name: "index_users_on_username"
     t.index ["username"], name: "index_users_on_username_trigram", opclass: :gin_trgm_ops, using: :gin
   end