Fields must render values in the current time zone.
This commit only contains tests because this ticket was just a symptom of
a regression from #18777 that was fixed separately.
Backport of 5444a9c from master.
This also updates all dependent functionality, including modelform_factory
and modelformset_factory, and the generic views `ModelFormMixin`,
`CreateView` and `UpdateView` which gain a new `fields` attribute.
Aymeric Augustin
Claude Paroz
Russell Keith-Magee
Luke Plant
Florian Apolloner