The input event handler's callback was not invoked inside an $apply()
block, so the $digest function was never called to propagate the changes
in vm.setEditText() in annotation.js. This happened to work previously
because ngModel's $setViewValue() was called to apply input changes,
which internally uses $scope.$apply().

This commit wraps the input events from the markdown editor with $apply
but debounces them. Since $apply() triggers every single watcher in the
application, running it on every keystroke causes noticeable lag
otherwise.

Fixes #3212