Rewrite Reference and ReferenceList fields on import

[cahrens]

Fixes STUD-149.

@dmitchell @singingwolfboy Please review. This branch is based off of #2503, so the first commit can be ignored (assuming you reviewed the other one!).

[cahrens]

Diffcover is lying-- the "Reference" code is covered by test_rewrite_reference (verified with breakpoint in debugger).

[singingwolfboy]

The spacing makes this hard to read. I'd format this more like:

if (original_location.tag == ref.tag and 
        original_location.org == ref.org and
        original_location.course == ref.course):
    new_ref = ref.replace(
        tag=target_location_namespace.tag,
        org=target_location_namespace.org,
        course=target_location_namespace.course
    ).url()

or

def locations_equal(a, b):
    return (a.tag == b.tag and a.org == b.org and a.course == b.course)

if locations_equal(original_location, ref):
    new_ref = ref.replace(
        tag=target_location_namespace.tag,
        org=target_location_namespace.org,
        course=target_location_namespace.course
    ).url()

[singingwolfboy]

Again, only nitpicks, so 👍. As for diffcover, you should probably file a bug report with @wedaly.

[dmitchell]

👍

[cahrens]

Filed https://edx-wiki.atlassian.net/browse/TE-346 for diffcover bug.