[1.6.x] Added missing release notes for older versions of Django

Backport of 3f6cc33cff from master
Fixed is_safe_url() to reject URLs that use a scheme other than HTTP/S.

This is a security fix; disclosure to follow shortly.
Apply autoescaping to AdminURLFieldWidget.

This is a security fix; disclosure to follow shortly.
Bumped version numbers for 1.6b2.
Added 1.4.6/1.5.2 release notes.
[1.6.x] Fixed overflow for the "Recent Actions" widget on the admin index.

Previously the CSS targeted "li.changelink" and therefore didn't
work for the "add" and "delete" actions.

Refs #14868.

Backport of 33fc083b0d from master
[1.6.x] Removed unneeded conditional_escapes from the testsuite.

Backport of 4e50e4065489f06d72eed8ce2d9ae98639be5a93 from master.
[1.6.x] Added some doc links for django.contrib.messages

Backport of b6178fa24b from master
[1.6.x] Updated docs following deprecation of django.views.defaults.shortcut

Follows 3f2befc

Backport of 354009d67e from master
[1.6.x] Added release date to 1.5 release notes.

Forwardport of 61283a8208 from 1.5.x
[1.6.x] Fixed #17778 -- Prevented class attributes on context from resolving as template variables.

Thanks KyleMac for the report, regebro for the patch, and Aymeric for the test.

Backport of 71b5617c24 from master.
[1.6.x] Fixed #20891 -- Removed part of the tutorial that requires pytz

Thanks AtomicSpark for the report.

Backport of 55339a7669 from master
[1.6.x] Fixed #20895 -- Made check management command warn if a BooleanField does not have a default value

Thanks to Collin Anderson for the suggestion and Tim Graham for
reviewing the patch.

Backport of 22c6497f99 from master
[1.6.x] Revert "Fixed #18491 -- deleting a proxy doesn't show warning about cascade deletes"

This reverts commit 2b48fcc607010065c0f8107baf669dd41b164f3c.

It introduced a regression (#20777) which we can't easily fix in 1.6.
[1.6.x] Updated translation templates
[1.6.x] Fixed #20829 -- Skip postgis metadata tables with introspection

Backport of 24088618 from master.
[1.6.x] Correctly format missing Pillow/PIL exceptions messages. refs #19934

Backport of b9590a6935 from master.
Fixed #20943 -- Weakly reference senders when caching their associated receivers

Backport of e55ca60903 from master.
[1.6.x] Fixed a test failure introduced in f0bc2865ff.

`classobj` objects cannot be weakly referenced on Python 2.6
[1.6.x] Fixed #20820 -- Model inheritance + m2m fixture loading regression

Tests by Tim Graham, report from jeroen.pulles@redslider.net.

Backport of 1ed77e7782 from master