Use custom image for default JupyterHub environment

We should create a custom image for our JupyterHub environment, that includes:

- The python libraries required for the notebooks, like `stac_ipyleaflet`, etc. so we don't need the `pip install` step inside the notebook (can leave it there for reproducibility, but then we don't have issues of needing to reload the notebook after installing, etc.)
- QGIS with some customizations - ideally we'd make some progress on our work over at https://github.com/2i2c-org/nasa-qgis-image and be able to include a custom project file in QGIS to pre-load the data layers we want

@sunu let's chat about this briefly before you're off - I can probably take care of this if you point me in the right direction.

cc @geohacker @zacharyDez