[1.5.x] Fixed #21002 -- Documented JSON session serialization requires string keys

Thanks jeroen.pulles at redslider.net for the report.

Backport of 3baf1d1042 from master.
[1.5.x] Fixed #20958 -- Documented that GenericForeignKey fields can't be accessed in forms.

Thanks marky1991.

Backport of 533d1ab334 from master
Fixed #19211 -- Adapted tutorial for Python 3

Backport of 7cc3acbb7 from master.
[1.5.x] Fixed #20900 -- Documented RemoteUserBackend.authenticate

Backport of 7b62b80693 from master
[1.5.x] Fixed #21047 -- Added CLA mesage on the new contributor advice doc

Backport of 93dd31cadf from master
[1.5.x] Fixed #20646 -- Clarified the use of AbstractBaseUser.REQUIRED_FIELDS

Thanks craigbruce.

Backport of db3de52807 from master
[1.5.x] Fixed #19295 -- Documented that CachedStaticFilesStorage isn't compatible with runserver --insecure.

Backport of 028db97503 from master
[1.5.x] Fixed #16992 -- Added InnoDB warning regarding reuse of AUTO_INCREMENT values.

Thanks kent at nsc.liu.se for the report.

Backport of c54fa1a7bc from master
[1.5.x] Fixed #20005 -- Documented that Oracle databases need execute permission on SYS.DBMS_LOB.

Thanks jafula for the suggestion.

Backport of a86ecc80a2 from master
[1.5.x] Fixed #20707 -- Added explicit quota assignment to Oracle test user
To enable testing on Oracle 12c
[1.5.x] Fixed #21075 - Improved doc for calling call_command with arguments.

Backport of fca4c4826e from master
[1.5.x] Prevented arbitrary file inclusion with {% ssi %} tag and relative paths.

Thanks Rainer Koirikivi for the report and draft patch.

This is a security fix; disclosure to follow shortly.

Backport of 7fe5b656c9 from master
[1.5.x] Added 1.4.7/1.5.3 release notes

Backport of baec6a26dd from master
[1.5.x] Bump version numbers for 1.5.3 security release.
[1.5.x] Bump version post-release.
[1.5.x] Documentation -- Improved description of cache arguments

- Fixed some grammar and formatting mistakes
- Added the type and default for CULL_FREQUENCY
- Made the note on culling the entire cache more precise. (It's actually
  slower on the filesystem backend.)

Backport of 5eca021d48 from master
[1.5.x] Fixed #20887 -- Added a warning to GzipMiddleware in light of BREACH.

Thanks EvilDMP for the report and Russell Keith-Magee
for the draft text.

Backport of da843e7dba from master