[ZEPPELIN-6340] Add ZeppelinEventBus and update NotebookServer to handle NoteRemoveEvent - #5441

WARNING conda.conda_pypi.main:notify_externally_managed_future(156): Did you know? You can install many PyPI packages with conda using the conda-pypi beta. Get started: https://docs.conda.io/projects/conda/en/stable/new-features.html