Additional logic to handle more course_image URL edge cases

This changes logic to allow more missed use cases of course_image to
work properly.  The cases are:

. XML courses with the course_image attribute set
. Mongo courses that are imported without a contentstore
. Mongo courses that have course_image set but don't have a content store

It also exports default images_static_course.jpg to
images/static_course.jpg to handle a use case where a course author
uploaded an image to the default location in studio without using the
studio interface for adding course images, they then export the course, and then
import it without a contentstore

common/test/data/toy/course/2012_Fall.xml

@@ -1,4 +1,4 @@
-<course>
+<course course_image="just_a_test.jpg">
     <textbook title="Textbook" book_url="https://s3.amazonaws.com/edx-textbooks/guttag_computation_v3/"/>
   <chapter url_name="Overview">
     <videosequence url_name="Toy_Videos">