Support displaying external resources in flow page

[jiaqing23]

Support displaying external resources like Jupyter Notebooks or NumPy/SciPy documentation in iframe. This would make it easier for students taking a ProctorU exam, as the Guardian browser only allows one tab, saving time spent switching and loading between different pages.

Example config: https://github.com/jiaqing23/relate-sample/blob/9a45091e4c00c4610060b205c3e805d74f071ade/flows/001-linalg-recap.yml#L43-L51

external_resources:

-
    title: Numpy
    url: https://numpy.org/doc/

-
    title: JupyterLab
    url: https://scicomp-jupyterlab.cs.illinois.edu/lab/