[1.7.x] Fixed #22502 -- Fixed microseconds/default/form interaction

Made explicit lack of microsecond handling by built-in datetime form
fields. Used that explicitness to appropriately nix microsecond
values in bound fields. Thanks Claude Paroz for the review.
Backport of a5de0df58 from master.
[1.7.x] Adjusted refactoring of vendor checks.

Thanks Shai for the thorough review.

Backport of fb90b7c from master
[1.7.x] Fixed #22508 -- Avoided overwriting select_related.

Previously, known related objects overwrote related objects loaded
though select_related. This could cancel the effect of select_related
when it was used over more than one level.

Thanks boxm for the bug report and timo for bisecting the regression.

Backport of f574220f from master
[1.7.x] Fixed #21685 -- Displayed app verbose name in admindocs model index

Backport of 4cf82e5bcf from master.