[1.5.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 1.5.2.
Added 1.4.6/1.5.2 release notes.
Bump version post-release.
[1.5.x] Added some doc links for django.contrib.messages

Backport of b6178fa24b from master
[1.5.x] Fixed link in 1.5.2 release notes
[1.5.x] Fixed #20444 -- Cookie-based sessions does not include a remote code execution-warning

Backport of d5ce2ff5e4 from master
[1.5.x] Fixed a rest mistake I introduced in d5ce2ff.

Backport of 753edfa4b5 from master
[1.5.x] Documentation - Noted that OneToOneField doesn't respect unique.

Added OneToOneField to the list of model fields for which the unique
argument isn't valid. (OneToOneFields are inherently unique, and if
the user supplies a value for unique it is ignored / overwritten.)
[1.5.x] Made description of LANGUAGE_CODE setting more clear.

297f5af222 from master.
[1.5.x] Fixed #20944 -- Removed inaccurate statement about View.dispatch().

Backport of bac4d03ce6 from master
[1.5.x] Documentation -- Clarified use of 'view' in test client introduction.

Backport of 2e926b041c from master
[1.5.x] Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

Backport of b0ce6fe656 from master
[1.5.x] Typos introduced in 57c82f909b.
[1.5.x] Fixed typo in docs/topics/conditional-view-processing.txt

Backport of 5fd2c979cb from master
[1.5.x] Fixed #20981 -- Noted the default value of disable_existing_loggers.

Backport of 095643e691 from master
[1.5.x] Minor spelling correction in ModelForms docs

Backport of 2fbf949760 from master
[1.5.x] Added links to file docs.

Backport of d72f83c410 from master