Bug fix for dragging past last element in list.

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.