This is a preparatory commit for using isort for sorting all of our
imports, merging changes to files where we can easily review the
changes as something we're happy with.
These are also files with relatively little active development, which
means we don't expect much merge conflict risk from these changes.
We add a new model, ArchiveTransaction, to tie archived objects together
in a coherent way, according to the batches in which they are archived.
This enables making a better system for restoring from archive, and it
seems just more sensible to tie the archived objects in this way, rather
the somewhat vague setting of archive_timestamp to each object using
timezone_now().
Tim Abbott
Mateusz Mandera