Cluster auto-discovery and management across environments

As a follow-up on #18 and #31, it would be nice if the "search button" and the cluster management section on the side panel could work across multiple (conda) environments.

Those features work very well when I'm using a single environment where everything (jupyterlab, dask, extensions, etc.) is installed.

However, my (possibly a common?) configuration consists of running jupyterlab from within its own dedicated, lightweight conda environment and using [nb_conda_kernels](https://github.com/Anaconda-Platform/nb_conda_kernels) to run kernels installed in other environments (one per project). In this case the search button is unresponsive and the cluster management section in the side panel only manages clusters in the jupyterlab environment. I could still manually copy dashboard addresses in the text field, though (and I'm happy doing this!)

Unfortunately, I have no idea on how much effort this would require to implement.