Datagrids allow columns to augment the queryset, pulling in additional

fields if needed. While we've generally gotten away with this, it

doesn't really work well in practice on Django 1.6, because calls to

select_related() do not chain. They've fixed this on newer versions of

Django, but we can't depend on those for this.
The good news is that they can be made to be chainable by merging the

select_related dictionaries on querysets. This format is standard on all

versions of Django 1.6 and higher.
This change introduces a chainable_select_related_queryset function that

takes in a QuerySet and returns a version supporting chaining. On Django

1.6, this returns a special subclass that handles merging. On newer

versions, this simply returns the provided QuerySet.
Datagrids make use of this to ensure that all select_related() calls

from queryset augmentation will stay performant and won't

unintentionally drop select_related() fields.

Unit tests pass.
Manually verified that different select_related() calls were being

factored in for the datagrid.