The `notifications`, `notification_history`, `templates` and `templates_history`
tables all had a check constraint on the postage column which specified
that the postage had to be `first` or `second` if the notification or
template was a letter. We now have two more options for postage -
`europe` and `rest-of-world`.
It's not possible to alter a check constraint, so the constraints have
to be dropped then recreated. We are not recreating the constraint on
the `notification_history` table since values here are always copied
from the `notifications` table.
The constraints get added as `NOT VALID` at first - this stage will lock
the tables, so updating the `notification` table and `templates` and
`templates_history` are done in separate migrations so that we don't lock
all tables at the same time. In a third migration we then run
`VALIDATE CONSTRAINT` for all tables - this will lock a row at a time,
not the whole table.
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