In Django 2.1, the preferred way to express a nullable BooleanField
changed from NullBooleanField to passing null=True to BooleanField.
This updates our codebase to use the preferred API. Tweaked by
tabbott to update the linter rules.
The migration is a noop for Django accounting only.
Part of #11341.
This directory contains scripts that are used in building, managing,
testing, and other forms of work in a Zulip development environment.
Note that tools that are also useful in production belong in
scripts/ or should be Django management commands.