Possible regression for img tags failing to render in Markdown cells

I came back to a Jupyter Notebook recently (I think after upgrading some Jupyter packages) and now images in my markdown cells that look like this fail to render:

Assuming of course that such an image exists in `img/python_logo.sg`. This approach is still working in JupyerLab. To get it working again I need to change the markup to:

Jupyter Notebook is rendering the following URL:

http://localhost:8889/edit/notebooks/img/python_logo.svg

When it should be this I think:

http://localhost:8889/view/notebooks/img/python_logo.svg

My current installed Jupyter libraries

```
$ pip freeze | grep jupyter
>>>
jupyter==1.0.0
jupyter-client==6.1.12
jupyter-console==6.4.0
jupyter-core==4.7.1
jupyter-packaging==0.9.2
jupyter-server==1.6.4
jupyterlab==3.0.14
jupyterlab-code-formatter==1.4.10
jupyterlab-pygments==0.1.2
jupyterlab-server==2.4.0
jupyterlab-widgets==1.0.0
````

Unfortunately I'm not sure what the previous environment that was working was as I'm using Binder, which doesn't like you pinning dependencies, and I'm struggling to navigate installed Jupyter deps in Binder. But that does mean I can point you in the direction of the Binder image where this is all working:

https://mybinder.org/v2/gh/ned2/melbviz/85ee2b64e0580c22ec6e34ce6b8b970b2726eab5?filepath=notebooks%2Ftalk_data_viz.ipynb

Reproduce the issue with the linked Binder image and a Markdown cell containing an img tag such as img/python_logo.svg. Compare the generated /edit/notebooks/... URL with the expected /view/notebooks/... URL, and inspect the notebook rendering path involved. Done means relative images render correctly in Markdown cells without requiring the /files/notebooks workaround.

Written by the indexing model from the issue text.