Fixed #23638 -- Prevented crash while parsing invalid cookie content

Thanks Philip Gatt for the report and Tim Graham for the review.
Fixed #23534 -- Added documentation on blocktrans tags

Mentioned that other block tags are not allowed inside
a blocktrans template tag.

Thanks to edu2004eu for reporting the issue.
Adapted invalid cookie test to all Python versions

Refs #23638. Older Python versions are less strict when parsing
invalid cookie content. The test just has to ensure Django doesn't
crash.