Support pulling from Google Drive or Dropbox for nbgitpuller

[yuvipanda]

jupyterhub/nbgitpuller#194 works on supporting pulling from arbitrary web sources, while preserving the merging behavior we have. It implements this in a pluggable fashion with pluggy. Google Drive and Dropbox (unauthenticated) are the first plugins

@sean-morris is working on this

TODO:

• Get Added non-git source puller functionality jupyterhub/nbgitpuller#194 merged
• Get Link generator handles non-git sources jupyterhub/nbgitpuller#195 merged

[choldgraf]

Update: I believe that this one is currently blocked on the two PRs that are currently open, but I don't think there's anything we can do to push those forward.

[choldgraf]

@yuvipanda what's the state on this one? I don't see much movement in those PRs, I'm assuming that we should just remove this one from our sprint plan? I'm also tempted to just remove it from the backlog in general, if it doesn't seem there is much urgency for it...

[yuvipanda]

@consideRatio has been doing amazing work here.