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.
Commit d4ee3023 and its parent have the history behind this code.
Since d4ee3023^, all new PreregistrationUser objects, except those for
realm creation, have a non-None `realm`. Since d4ee3023, any legacy
PreregistrationUsers, with a `realm` of None despite not being for
realm creation, are treated as expired. Now, we ignore them
completely, and remove any that exist from the database.
The user-visible effect is to change the error message for
registration (or invitation) links created before d4ee3023^ to be
"link does not exist", rather than "link expired".
This change will at most affect users upgrading straight from 1.7 or
earlier to 1.8 (rather than from 1.7.1), but I think that's not much
of a concern (such installations are probably long-running
installations, without many live registration or invitation links).
[greg: tweaked commit message]
Tim Abbott
Anders Kaseorg
Rishi Gupta