* Changed update_fact_billing DAO function to update the table with the
real data for postage instead of hard-coding in 'second'.
* Added a test for the create nightly billing task to test that rows
with different postage are being inserted correctly.
Removed the occasionally failing test to check how ft_billing upserts
postage data. This test will be re-added once the postage column has been
added to the primary key.
* Updated the 'fetch_billing_data_for_day' DAO function to take postage into
account
* Updated the 'update_fact_billing' DAO function to insert postage for
new rows. When updating rows which are identical apart from the postage, the
original row will be kept. (This behaviour will change once postage is
added to the primary key - at this point, upserting will add a new row.)
* Also changed some fixtures / test set up functions to take postage
into account
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.
