Dispatch to custom exporter in ScriptExporter

The logic previously implemented in the menubar javascript is moved to the ScriptExporter itself.

This removes the need for Javascript to interpret `nbconvert_exporter` metadata at all, and ensures `--to script` is consistent with `Download as...script`, etc.

A side effect is that there should be no reason to use `--to python` anymore.
If it uses a custom exporter, these fields (file_extension, output_mimetype) aren't used, so I'd move these lines below the `if` block to make that clearer.