Build conda pkgs with conda-forge

### Proposed change
It would be useful if batchspawner could be installed from the conda-forge conda channel.

### Alternative options
People can pull batchspawner conda packages from https://anaconda.org/hcc/batchspawner but a 'JupyterHub' ecosystem-branded package would be more reassuring.

### Who would use this feature?
Anyone wanting to install batchspawner using conda.

### (Optional): Suggest a solution
Copy and adapt the conda-forge feedstocks used for other JupyterHub Spawner implementations - see https://github.com/conda-forge?q=jupyterhub&type=&language=&sort=