[1.7.x] Fixed #23587 -- Clarified admin template overriding in tutorial 2.

Thanks Petrus van Bork for the report.

Backport of e949f4435f from master
[1.7.x] Fixed #23333 -- Made urlsafe_base64_decode() return proper type on Python 3.

Backport of 03d89168a2 from master