Colon in feedback path conflicts with jupyter-fs

[jeflem]

The path to feedback HTML files contains a timestamp and, thus, a colon. If the jupyter-fs lab extension is installed next to nbgrader, colons are not allowed in paths (the colon is used internally by jupyter-fs to prepend a 'drive' to paths). Navigating to the feedback files in JupyterLab users get an error message, that the path does not exist.

This issue could be solved on jupyter-fs side, but since colons in path should be avoided for other reasons, too (see this discussion on stackexchange for instance), nbgrader maybe should avoid colons in paths.