Set NBGITPULLER_PARENTPATH to work

Currently nbgitpuller pulls the repos to the home directory, ie it will not persist over reboots of the notebook. To create the links: https://jupyterhub.github.io/nbgitpuller/link.html eg. https://hub.cse.kau.se/hub/user-redirect/git-pull?repo=https://git.cse.kau.se/jonakarl/test&urlpath=lab/tree/test/&branch=master

TODO:

• NBGITPULLER_PARENTPATH should be set to "work" so the work is saved in their work folder (ie on nfs)

Workaround (https://jupyterhub.github.io/nbgitpuller/topic/url-options.html) : add &targetPath=work/<REPONAME> to url