Allow relative paths in navigation in config.yaml or allow transparently using symlinks

[apirogov]

Currently the only way to have the same page appear in both learners and instructors view appears to be duplicating the content.

A clean solution would be either to allow something like:

learners: 
- acknowledgements.Rmd

instructors: 
- ../learners/acknowledgements.Rmd

or allowing to use a symlink in place of the file.

In both cases currently the validation fails complaining that the file does not exist.

[zkamvar]

Thank you for submitting this report. I believe this is similar to the concerns raised in #394 in which we proposed to include all learner material in the instructor dropdown so that they can access it readily for their learners.

I think your suggestion could indeed be a solution that would allow lesson authors to arrange content that instructors would want to share with learners.

One of the things about The Workbench is that all files are built for the instructor and learner views, so even though the navigation does not exist, the files do.

I don't have an answer now, but I will see what I can do when I take care of the current PR backlog.