#15185 -- list_display_links=None to get no links in change list grid

#15185 (https://code.djangoproject.com/ticket/15185)

Allow ModelAdmin.list_display_links to be None so gent on links in the admin change list view grid.

I'm still not 100% convinced about the smell of allowing a variable take either None or a list/tple value. But we have a precedent with the 'actions' option.