Use default dict for default job stats
Use default dict for default job stats

This is just some refactoring.

`defaultdict` is a data structure which won’t raise a `KeyError` if you
try to access a key that doesn’t exist.

By passing `int` as the first argument, trying to access the value of
any key that doesn’t exists will return the value of `int()`, ie `0`

gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin