Support deploying python files in JupyterLab

[mmarchetti]

The JupyterLab plugin in #234 supports deploying notebooks, but not Python source files (that might contain deployable apps).

We use an INotebookTracker that notifies us when the active notebook changes. There is a similar notifier for file editors. We should consider scanning the source for imports that indicate a valid app (like the agent does during content type detection) so we're not constantly offering to publish people's library code, only to have it fail.