Make display for null fields in Django admin configurable through settings

[Loek van Gent]

Display value for null fields is '-' now. It would be nice if this is configurable in settings. '-' can be a bit hard to click if you use it in a list filter.

This relates to changes made in https://code.djangoproject.com/ticket/24226.

​​http://stackoverflow.com/questions/18324030/how-to-change-empty-changelist-value-in-django-admin
​​http://stackoverflow.com/questions/28174881/in-the-django-admin-changelist-how-do-i-display-a-blank-space-instead-of-the-de?noredirect=1#comment44719629_28174881

[Tim Graham]

By settings, I think attributes of ModelAdmin and AdminSite would probably be the way to go.

[Tim Graham]

Reviewed PR.

[Loek van Gent]

Applied suggested changes. PR ready for new review.

[Tim Graham <timograham@…>]

In 72f769f4:

Fixed #24474 -- Allowed configuring the admin's empty change list value.

[Tim Graham <timograham@…>]

In 1273a7a0:

Revert "Fixed #24474 -- Allowed configuring the admin's empty change list value."

This reverts commit 72f769f494822981db6df9524b92a2d86f8e69fe.

There are several test failures that need to be fixed.

[Loek van Gent]

I see the failing tests now. I'll look into this.

[Loek van Gent]

Django Sprint Cardiff! I'm continuing this ticket now.

[Tim Graham]

Looks good, pending a cosmetic comment.

[Loek van Gent]

Do you mean this one?
​https://github.com/gannetson/django/commit/fcb527df09a8087763725e123255eebca2474e76

[Tim Graham <timograham@…>]

In 0207bdd:

Fixed #24474 -- Allowed configuring the admin's empty change list value.

[GitHub <noreply@…>]

In 67a1ae6:

Refs #24474 -- Changed AdminSite.empty_value_display property to an attribute.

This was implemented with a property getter and setter when introduced
in 0207bdd2d4157c542c981264c86706b78ca246e9.

There is nothing special occurring here though - a simple read from and
assign to the underlying private attribute.