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.
Merge branch 'master' of git://github.com/django/django into ticket_21936