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>

This will help cut down on the amount of manual intervention required when setting up SPOCs and re-runs.