Distribute datasets as artifacts

[johnnychen94]

CRef: #57 (comment)

We currently use DataDeps as an interface to download datasets from original websites. While it's good to give a clear license and source, it can be unstable for reproducibility because worldwide users might have difficulties connecting original sites. The original sites might also be offline for various reasons, e.g., #57.

To avoid issues like #57 in the future and accelerate dataset downloading, we could take advantage of Julia's Artifacts system and let Pkg/Storage servers hold and distribute the datasets. MLDatasets don't hold large datasets so it adds little stress to the Julia ecosystem.