Add indexes: user_answers_created_at_user_id_question_number, user_answers_created_at_user_id

[Hausekey]

Fixes #136

Creates 2 indexes on table user_answers. One with columns created_at, user_id, question_number. The other with created_at, user_id. To validate, please ensure code matches style guidelines, runs without errors, and creates the indexes.