I originally thought `SET_NULL` would be a good way to "do stuff later", but
that's only so the degree that [1] updates are cheaper than deletes and [2]
2nd-order effects (further deletes in the dep-tree) are avoided.
Now that we have explicit Issue-deletion (deps-first, delayed, properly batched)
the SET_NULL behavior is always a no-op (but with cost in queries).
As a result, in the test for issue deletion (which has deletes for many
of the altered models), the following 8 queries are no longer done:
```
SELECT "issues_grouping"."id", [..many fields..] FROM "issues_grouping" WHERE "issues_grouping"."id" IN (1)
UPDATE "events_event" SET "grouping_id" = NULL WHERE "events_event"."grouping_id" IN (1)
[.. a few moments later..]
SELECT "issues_issue"."id", [..many fields..] FROM "issues_issue" WHERE "issues_issue"."id" = 'uuid'
UPDATE "issues_grouping" SET "issue_id" = NULL WHERE "issues_grouping"."issue_id" IN ('uuid')
UPDATE "issues_turningpoint" SET "issue_id" = NULL WHERE "issues_turningpoint"."issue_id" IN ('uuid')
UPDATE "events_event" SET "issue_id" = NULL WHERE "events_event"."issue_id" IN ('uuid')
UPDATE "tags_eventtag" SET "issue_id" = NULL WHERE "tags_eventtag"."issue_id" IN ('uuid')
UPDATE "tags_issuetag" SET "issue_id" = NULL WHERE "tags_issuetag"."issue_id" IN ('uuid')
```
(breaks the tests b/c of constraints and not always using factories; will fix next)
Klaas van Schelven