Lazily fetch django user only on first load

[esoergel]

Product Description

This is a performance improvement

Technical Summary

That call to user.get_django_user() takes something like 2.5 seconds on prod, since it does an inexact match:

        return queryset.get(username__iexact=self.username)

@gherceg is investigating whether it would be possible to switch to an exact match, which is much much faster. Meanwhile, it'd be better to skip the lookup regardless, and it turns out that's easy to do. From the docs:

If there are any callables in defaults, evaluate them

Feature Flag

Safety Assurance

Safety story

This is a trivially simple change, and I believe test coverage is sufficient (though I'm verifying the performance improvement on staging)

Automated test coverage

QA Plan

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change