Allow expressions to be used in order_by queryset method

[Josh Smeaton]

Now that expressions have landed, we should try to expand the scope of queryset methods where they are allowed as arguments. There is a proof of concept for allowing order_by to accept expressions here: ​https://groups.google.com/forum/#!topic/django-developers/arR7B_Luszc

I'm marking this as 1.8, as I plan to clean up the patch and have it ready before the alpha. If it doesn't make it in time, we can change the version.

[Anssi Kääriäinen]

One case to be aware: QuerySet.reverse(). This should work for any custom expression order_by, too. This means that if .order_by(NullsLast('somecol')) produces SQL like:

   SELECT ... FROM ... ORDER BY somecol ASC NULLS LAST

then .order_by(NullsLast('somecol')).reverse() should produce SQL like:

   SELECT ... FROM ... ORDER BY somecol DESC NULLS FIRST

[Josh Smeaton]

​https://github.com/django/django/pull/3868

[Josh Smeaton <josh.smeaton@…>]

In 21b858cb6735cdfdc695ff7b076e4cbc1981bc88:

Fixed #24060 -- Added OrderBy Expressions

[Josh Smeaton <josh.smeaton@…>]

In f218a2ff455b5f7391dd38038994f2c5f8b0eca1:

Refs #24060 -- Added a test demonstrating reverse order isn't mutable

[Josh Smeaton <josh.smeaton@…>]

In 405351ba2f639a1a0c092ecb6d93f9f7a646686a:

[1.8.x] Refs #24060 -- Added a test demonstrating reverse order isn't mutable

Backport of f218a2ff455b5f7391dd38038994f2c5f8b0eca1 from master