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

@sarina This is another static_asset_path related fix. It again is touching in a lot of places, but based on git blame I would think it would be something for maybe @chrisndodge and @cpennington to look at.  There is an odd choice to force XMLModuleStore courses to use one URL for their course_image even if it is set in the XML that I changed and might cause some issues. I can separate that out from the static_asset_path use case if needed.