Avoid unnecessary calls to get_parent_xblock in _course_outline_json

When calculating _course_outline_json's `xblock_studio_url(xblock)` we hit Mongo database often, but mostly because methods used in `xblock_studio_url` (`xblock_has_own_studio_page`, `is_unit`) are calling `get_parent_xblock` a lot. This change tries to reuse information about parent_xblock when possible.

Reduces rendering time for Justice course from 5.5 to 3.2 seconds for me.

review: @cahrens @dan-f
> @cahrens Why not just "if parent_xblock"?

I wanted to emphasize the fact that it parent_xblock is not surely False (there are no parent) but that this information is unknown to us at the moment.