Tidy up "format" method for monthly letter usage
Tidy up "format" method for monthly letter usage

This did more than it said and had some unconventional behaviour:

- It modified the input data. We can avoid this by computing the
postage group on-the-fly and using "sorted" instead of "sort".

- It defined a custom, named tuple. This isn't necessary as Jinja
allows us to access elements by qualification (".") already.

We can also use the same lambda function to group and sort items,
since the sort predicate is the same one we use to group them.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin