Add download activity for conda (in addition to PyPI)

[neuromusic]

per @haesleinhuepf in #500 (comment)

Consider adding condastats as data source as well. The picture drawn from conda download > statistics and pypistats is not 100% consistent -> scientifically very interesting ;-)

See also

• https://github.com/haesleinhuepf/napari-pypistats/blob/main/conda_download_stats.ipynb

[neuromusic]

demo notebook here: https://github.com/haesleinhuepf/napari-pypistats/blob/main/conda_download_stats.ipynb