[1.7.x] Fixed #23597 -- Clarified the manager that {Single,Multiple}ObjectMixin.model uses.

Backport of e501d4c505 from master
[1.7.x] Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

Backport of a24cf21722 from master
[1.7.x] Updated links in admin docs to use :ref:

per Sphinx docs: "Using ref is advised over standard reStructuredText
links to sections because it works across files, when section headings
are changed, and for all builders that support cross-references.

Backport of 13409b932a from master
[1.7.x] Fixed #23594 -- Fixed deepcopy on ErrorList.

Thanks Troy Grosfield for the report and Tim Graham for the tests.

Backport of ec2fd02bb3 from master
[1.7.X] Fixed #23602 -- Add comment on get_absolute_url regarding user input

Backport of 04bd84786d39b8a17620dfb3b354599d8d95417b from master.
[1.7.x] Fixed #23601 -- Ensured view exists in URLconf before importing it in admindocs.

Backport of 2f16ff5a6c from master
[1.7.x] Fixed #23593 -- Fixed crash in AdminEmailHandler with non-ASCII characters in request.

Thanks edevil for the report and Simon Charette for review.

Backport of 9dff5ce7c7 from master
[1.7.x] Fixed #23607 -- Typo in docs/ref/contrib/staticfiles.txt.

Thanks Rolandde for the report.

Backport of 87eedfff44 from master