Feature/cdodge/convert to portable links on import and clone

This will do rewrites of two known unportable linking formats when importing and cloning:

/c4x/[org]/[course]/asset/[name] --> /static/[name]

and

/courses/[org]/[course]/[run]/jump_to/i4x://[org]/[course]/[category]/[name] --> /jump_to_id/[name]

and

/coursers/[org]/[course]/[run]/ needs to be substituted with the target course-id

This will help cut down on the amount of manual intervention required when setting up SPOCs and re-runs.