Follow-up of d97cce34 and 7d3fe36c.

Follow up to ac59ec8f.

The implementations have unintentionally diverged.

Fixed #27629 -- Added router.allow_relation() calls for assignments between unsaved model instances.

Thanks Tim Graham for the review.

The problem:
“3 days ago” should translate to “vor 3 Tagen” in German, while “3 days” translates to “3 Tage”. #21408 describes that django always translated to “Tage”, even when the dative “Tagen” was correct. The same applies to months (“Monate”/“Monaten”) and years (“Jahre”/“Jahren”).

The solution:
Let `timesince` caller provide the string dict to use for the time-related strings.

Thanks Tim Graham for the review.

Fixed #28913 -- Fixed error handling when MIGRATIONS_MODULES specifies a nonexistent top-level package.

* Added logging of 500 responses for instantiated responses.
* Added logging of all 4xx and 5xx responses.

Thanks Tim Graham and Mariusz Felisiak for the reviews.

Fixed #29367 -- Fixed model state on objects with a primary key created with QuerySet.bulk_create().

COUNT is more expensive than EXISTS; use the latter when possible.