Remove broken Codepen CSS URLs from all sample docs and replace for external_stylesheets example

[rpkyle]

As of this week, the Codepen URL we have frequently included as CSS boilerplate in sample apps is no longer accessible:

This is already causing friction for users, as evidenced by https://github.com/plotly/dash/issues/1410.

We should remove https://codepen.io/chriddyp/pen/bWLwgP.css from any apps that currently use it, so that they deploy/run successfully.