The dropdown toggle in the MenuButtonView splitbutton would let click

events propagate up, and depending on what child node of the toggle was

being clicked, the modal library from Djblets could interpret it as a

request to close the modal.
This is where Djblets handles the click event.
This patch stops click events on the toggle from propagating, so the

Djblets library never has a chance to get confused.
Fixes bug #4944.

Local manual testing on the Review editor publish splitbutton. Ran all

JS unit tests, and wrote a new unit test for this particular case.