Updated tutorial 1 to reflect changes in default project template.

Thanks JonLoy for the patch.
Fixed #19044 -- Made `DeletionMixin` interpolate its `success_url`.

Thanks to nxvl and slurms for the initial patch, ptone for the review
and timo for the documentation tweaks.
Added a check in the creation of IS NULL clauses.

value_annotation isn't very well defined. Before this change, setting it
to datetime.datetime could silently reverse the behavior of isnull
lookups. This commit doesn't have any consequences on the current code.
It's just a safeguard for future ORM hackers.