Added the letter_rate table to the list of tables which does not get
deleted after each test run and changed the tests to use the real letter
rates.
Also removed the letter rate DAO since this was only being used in
tests, so was no longer needed.
`created_at` was added previously and made nullable temporarily. This
commit now populates the column, ensures that it will always have a
value, and makes `created_at` non-nullable.
