Fixed #21460 -- Reenabled proper template precedence in find_template

Refs #20806. Thanks Unai Zalakain for the review.
Fixed missing admindocs' site_header
Merge pull request #1877 from Bouke/patches/4

Fixed missing admindocs' site_header, refs #21293
Fixed sentence structure in docs/internals/contributing/triaging-tickets.txt
Fixed #21487 -- Session cannot store tzinfo instances anymore.

Thanks filipp for the report.

Forward-port of 1eddca0a from stable/1.6.x.
Fixed some flake8 issues
Fixed #21491 -- Removed documented workaround for a known issue.

The issue was that two M2M hidden reverse managers
(related_name ending with a '+') could clash with each other.

Refs #21375 and #15932. Thanks Baptiste.
Merge pull request #1965 from loic/ticket21491

Fixed #21491 -- Removed documented workaround for a known issue.
Fixed #21443 -- Cannot show debug info on PY3's importlib

Thanks productions@zaziork.co.uk for the review.
Removed unused import in docs/topics/http/sessions.txt
Noted that localmem is the default cache.
Improved docs for ModelFormSet.clean().