Add users remember_token index

[abcang]

If remember_token is nil and devise remember_me is executed, generate remember token and run SQL to check if it is unique.

SELECT  "users".* FROM "users" WHERE "users"."remember_token" = 'xxxxx' ORDER BY "users"."id" ASC LIMIT 1

https://github.com/plataformatec/devise/blob/v4.7.1/lib/devise/models/rememberable.rb#L150

However, remember_token has no index, so the more users, the longer it takes to check. This problem happens when logging in for the first time and after logging out. Adding an index to remember_token prevents it from slowing down.