Bug fix for dragging past last element in list.

[cahrens]

STUD-879

This also makes dragging up into the beginning of a list easier.

Note that I had to add margins (5 px) to the subsections in the unit test to space things apart a bit. Previously in the test, each unit ended at the same place where the next unit began. This made testing the before/after edge cases impossible. In our application, we have quite a bit of space around subsections and sections, so adding margins to the test cases seemed reasonable to me.

@andy-armstrong Please review the code and check out the branch (git checkout christina/dnd) to manually test it.

Sorry about the whitespace changes in the spec-- IDE reformatting.

I advise adding ?w=1 to the end of the URL to ignore whitespace differences.

[andy-armstrong]

The logic looks reasonable, and my testing all worked. Nice! 👍

P.S. It seems like a lot of logic specifically for the overview component. Have we thought about how to make this into shared draggable logic that can be used by other components. Or is overview.js already used in a shared way?

[cahrens]

overview.js is not really shareable at this point (beyond its use on both the overview page and the subsection edit page). The draggabilly library is the reusable part.

No, thought has not been given to how this code code be made more reusable. This was our first foray into dragabilly. The next time we make something draggable, I agree we should look at this code to see what could be pulled out.

[cahrens]

Another thing--I am going to let Frances get her PR reviewed and merged first (https://github.com/edx/edx-platform/pull/1896). Then I want to make sure my code still works.

[cahrens]

I'm closing this because I now have a branch on top of Frances' changes.