move DEFAULT_STATIC_FILES_PATH to frontend.html.notebook

and move the friendly version checks to notebookapp.py

DEFAULT_STATIC_FILES_PATH is now accessible without pyzmq/tornado/jinja being importable. If someone tries to use old pyzmq or tornado directly with handlers, the version check won't happen, but that's probably the right thing to do anyway.

I also cleaned up a few workarounds for old pyzmq that we no longer support.

Question: should we move `check_for_zmq` to somewhere generic, like `utils.version`?