Fixed failing test around DST change.

The timezone arithmetic done in JS can be off by one hour around DST
change. We work around this issue by adding one extra hour to the test
error margin when we detect a DST change is near.

Refs #20663.
Merge pull request #1853 from loic/dst

Fixed failing test around DST change.
Re-expose Count here
Whitespace fix
Fixed #20849 -- ModelForms do not work well with prefetch_related.

model_to_dict() (used when rendering forms) queries the database
to get the list of primary keys for ManyToMany fields. This is
unnecessary if the field queryset has been prefetched, all the
keys are already in memory and can be obtained with a simple
iteration.
Fixed regressions from 36ded01527b690b5df0574492af9cfcc2ea3d1dc.

Refs #21302