Fixed a KeyError on login with legacy sessions; refs #21649.

Thanks Loic for the report.
Fixed #22444 -- Marked initial SQL/fixture loading as deprecated.

Thanks Karen Tracey for the report.
Fixed #22454 - Changed compatibility warning hint

The warning hint of `_check_test_runner` of 1.6 compatibility had a link
to a general release note. The link should be edited to refer the
relevant "Backwards incompatible changes in 1.6" section that documents
the cause and the possible solutions and workarounds of the warning.
Revert "Fixed #22397 -- Issues removing M2M field with explicit through model."

This reverts commit 00e3b9a2a992ee0b7288eeeb03e7cbd52ebc6dce.

It's causing a regression when tested with the proxy_model_inheritance tests.
Fixed #22464 - included contrib/gis/static in tarball

After switching to setuptools in commit 66f546b,
contrib/gis/static isn't included in tarball anymore, this commit adds
that path to be `recursive-include` in `MANIFEST.in`
Merge pull request #2586 from mmardini/patch-2

Fixed #22464 - included contrib/gis/static in tarball
Fixed a test failure introduced by 214d1e1b0f.
Fixed #22465 -- New assertion assertJSONNotEqual
Added missing versionadded annotation and tweaked release note; refs #22465.
Fixed #22471 -- Corrected misprint in i18n docs

Thanks to aruseni for the report.
Added some missing items to the 1.6.3 release notes.
Fixed #22399 -- Forced fixture reading in binary mode

This might help on systems where default encoding is not UTF-8 (and
on Python 3).
Thanks bacilla for the report.
Fixed #22472 -- Clarified meaning of dots in tutorial 4 code block.
Adapted fixture read mode to file type

Binary mode added in ed532a6a1e is not supported by ZipFile.
Refs #22399.
Fixed #22473 -- Emphasized that Model.clean() is not called from Model.save().
Fixed #22458 -- Added a note about MySQL utf8_unicode_ci collation

Thanks tobami at gmail.com for the report.
Moved RemoteUserBackend documentation to reference guide.
Fixed #22397 -- Issues removing M2M field with explicit through model

Changed the migration autodetector to remove models last so that FK
and M2M fields will not be left as dangling references. Added a check
in the migration state renderer to error out in the presence of
dangling references instead of leaving them as strings. Fixed a bug
in the sqlite backend to handle the deletion of M2M fields with
"through" models properly (i.e., do nothing successfully).

Thanks to melinath for report, loic for tests and andrewgodwin and
charettes for assistance with architecture.
Fixed #22417 -- Added additional documentation for refs #16969.

Thanks Jon Foster for the report.
Changed paths in docs that referred to Python 2.