Fix explainer dashboard (custom wrapper images)

[DannyDannyDanny]

• 📝 note on branch name: The branch name is called add/logging as this PR was originally scoped to add logging from the explainer-dashboard-threads that listener.py spins up. This PR does not actually add this functionality. In the distant future we might want to add logging to explainer dashboard to achieve logging from threads.
• ✨ added: this PR adds the following functionality:
  • notebook server: submit_dashboard now checks if the model (or its wrapper) is locally defined and, if it is, writes out a DASHBOARD_NAME.syspaths file (alongside the DASHBOARD_NAME.yaml and DASHBOARD_NAME.joblib files which it already produces). The .syspaths file contains the path to custom python modules used to define the explainer.model in the explainer dashboard (which is passed to submit_dashboard). More details in this trello comment.
  • notebook server: listener.py now checks for the existence of a DASHBOARD_NAME.syspaths (alongside the DASHBOARD_NAME.yaml and DASHBOARD_NAME.joblib files which it already looks for). If a syspaths file is found, read path from it and add to sys.path.
• 📣 up next: example notebooks / cleanup
• 🚧 this PR is still being written