The `notifications.id` column is the most probable column to run out of
values. This is because it is an `int` column that has only 2147483647
values and many notifications are generated on a regular basis in an
active community. This commit migrates the column to `bigint`.
These migrations do not use `ALTER TABLE ... COLUMN ... TYPE` in order
to avoid the `ACCESS EXCLUSIVE` lock on the entire table. Instead, they
create a new `bigint` column, copy the values to the new column and
then sets the new column as primary key.
Related columns (see `user_badges`, `shelved_notifications`) will
be migrated in a follow-up commit.
