this replaces content_char_count, by performing the additional
steps to calculated billable units at insert time, rather than
read time. This means we can take into account whether the
service was in research mode or using a test api key when the
notification was sent :tada
use NotficationHistory instead. Unfortunately this means the SQL
gets a bit gnarly, as we have to repeat notifications_utils'
`get_sms_fragment_count` functionality inside a SELECT 😱
