Fixed #22064 -- Add check for related_name

Validates that related_name is a valid Python id or ends with a '+' and
it's not a keyword. Without a check it passed silently leading to
unpredictable problems.

Thanks Konrad Świat for the initial work.
Fixed #23597 -- Clarified the manager that {Single,Multiple}ObjectMixin.model uses.
Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.
Fixed #23601 -- Ensured view exists in URLconf before importing it in admindocs.
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.
Added my bio to the core team.
Fixed #23594 -- Fixed deepcopy on ErrorList.

Thanks Troy Grosfield for the report and Tim Graham for the tests.
Fixed #23602 -- Add comment on get_absolute_url regarding user input
Merge pull request #3307 from Markush2010/ticket23602

Fixed #23602 -- Add comment on get_absolute_url regarding user input
Moved release note for refs #23601 to 1.7.1.
Merge pull request #3308 from aericson/ticket_22064

Fixed #22064 -- Add check for related_name
Fixed #23593 -- Fixed crash in AdminEmailHandler with non-ASCII characters in request.

Thanks edevil for the report and Simon Charette for review.
Fixed #23607 -- Typo in docs/ref/contrib/staticfiles.txt.

Thanks Rolandde for the report.