Fix/remove `robots.txt` and `sitemap.xml`

[lgeiger]

Our server currently serves robots.txt and sitemap.xml.

nteract.io/server.js

Lines 10 to 20 in 48f98e0

	if (!dev) {
	/**
	* Static Files
	*/
	server.get('/robots.txt', (req, res) => {
	return res.sendFile('/static/robots.txt');
	});
	server.get('/sitemap.xml', (req, res) => {
	return res.sendFile('/static/sitemap.xm');
	});
	}

Both files aren't generated at the moment. We should either remove this behavior or correctly generate them.

[rgbkrk]

sitemap.xm - well there's a typo

We could create robots.txt. I like serving humans.txt too.