perf: instance cache and select-related for license serialization

[iloveagent57]

Description

For large numbers of licenses (> 1000), this change gets us from 100s of SQL queries down to around 10 for each page of admin-license-list results.

1. Select/prefetch related plans, customer agreements, and renewals in the base queryset of the viewset.
2. add @cached_property to net_days_until_expiration on the agreement model.
3. ...but, (2) alone is not enough, because the license serializer is going to instantiate a new instance of a CustomerAgreement per license (but because of (1), it doesn't go to the DB to hydrate it). So we need to roll our own instance cache of MinimalCustomerAgreementSerializer.net_days_until_expiration.
4. For good measure, throw an instance cache onto the viewset's _get_subscription_plan() - this eliminated 4 duplicate queries.

Testing considerations

• Include instructions for any required manual tests, and any manual testing that has
  already been performed.
• Include unit and a11y tests as appropriate
• Consider performance issues.
• Check that Database migrations are backwards-compatible

Post-review

Squash commits into discrete sets of changes

[pwnage101]

Does this class-var-based memoization approach work even if many=True is passed to this serializer? I'm not suggesting we actually use many=True anywhere currently, but it does feel like one of those hidden time bombs if it doesn't work.

[pwnage101]

Just one question, but the rest LGTM!

[iloveagent57]

closing for now, trying new approach to the nested serialization