Fixed #24893 -- Fixed lack of unique constraint when changing a field from primary_key=True to unique=True

Thanks Tomáš Ehrlich for help with the patch.

Optimise tutorial images

Forwardport of 2456276b from stable/1.8.x

Nested deconstruction should recursively deconstruct items within list,
tuple and dict values.

QuerySet.exists() incorrectly handled query.group_by = True
case (grouping by all select fields), causing GROUP BY
expressions to be wiped along with select fields.

Thanks to Tim Graham for the report.