Clean up OLX

MNG-369

Studio exports OLX that consists of as many tiny files as possible (why?!). This is hard to edit by hand, and doesn't show nice diffs on GitHub.

Here I have consolidated the OLX so that there is one file per sequential (subsection). This will produce much nicer diffs in the future and be way easier to edit by hand, especially when moving things around. However, it does mean that editing in Studio and then exporting the course would not be supported as that would undo this work. Personally, I find the hand-editing workflow easier when the changes must be reviewed on GitHub (e.g. you never need an "Export" step; you simply push the changes to GitHub and use its auto-generated tarball link).

For now I've only tried this out with the "First Sprint" subsection.

Tarball link: https://github.com/open-craft/courses/archive/onboarding-cleanup.tar.gz

Preview link: https://courses.opencraft.com/courses/course-v1:OpenCraft+onboarding-dev+course/courseware/chapter_newcomer/first_sprint/?child=first (I have already imported the new tarball, but there's not much to see - there should be no visible changes).

This won't change any URLs.

To keep this review simple, I have not changed any content at all - just the format.