WIP: enable dask-gateway

[bolliger32]

Workflow

• Notebook:worker pairing builds & passes local build, visual inspection & client.map tests
• Passes travis tests
• Image deployed on dev branch (see notebook and worker dev tags)
• Worker passes manual deployment test on compute.rhg
• Notebook+worker passes test-hub deployment
• Updates integrated into downstream images

Summary

This PR is designed in concert with RhodiumGroup/helm-chart#14 and RhodiumGroup/rhg_compute_tools#87 and should be available to be tested on the adrastea cluster (which is set up with a dask-gateway server). In the process of incorporating gateway, I shifted some structure around a bit based on updates that had been incorporated in the coastal image workflow. See Features for details

Features

• incorporated dask-gateway
• moved environments and other resources shared across worker and notebook images into a shared_resources folder so that they are easily copied into each image's subfolder
• added a scheduler image which is just a lightweight version of our images that will be used for dask-gateway schedulers
• added some jupyterlab extensions just for some improved UI. in particular
  • TOC
  • jupyterlab-code-formatter (in-notebook application of black/isort)
  • git
  • An improved CSV tool
  • data explorer (a la RStudio)
  • added some rulers
• closes Install NBDIME notebook diff plugin #133