Move external frontend dependencies above internal
Extend the Annotator.Threading prototype

With the messageThread methods. This allows methods to be overridden
by the object itself.
Implement a simpler pruneEmpties method

This will now always keep the branch unless the thread has no
message and no children.
Reorder Karma includes to include vendor first
Add unit tests for new pruneEmpties behaviour
Merge pull request #1711 from hypothesis/1513-fix-holes-in-threads

Fix holes in message threads