Fix view feedback link when root_dir is set

[brichet]

The view feedback link does not work when the root_dir of JupyterLab is different from './'. The reason is that the feedback path is not relative to the root_dir but to the directory where nbgrader started.

This PR solves the problem by changing the feedback path to a relative path from root_dir.

I observed the same issue with classic notebook (in version \0.7.x) if the notebook_pathis different from './'. This PR should probably be back ported to0.7.x` branch.

Related to https://github.com/jupyter/nbgrader/issues/1664

[github-actions[bot]]

:point_left: Launch a Binder on branch brichet/nbgrader/fix/view_feedback

[brichet]

@meeseeksdev backport to 0.7.x

[perllaghu]

I can confirm this branch works for my docker notebook images, which are deployed in a K8s cluster.