Downloading notebooks no matter what

[cccs-cat001]

My attempt at fixing this issue: #11

I'm not great at javascript, but I think this will just download the notebooks when the directory exists too.

[jeffyjefflabs]

As I said on #11, this was not intended to be a recurring "sync" sort of feature, just a one-time download at startup. I'm not opposed to changing that (I'll defer to @mcrutch), but a couple concerns:

• It may be different in Lab, but the Classic version of the extension actually gets triggered fairly often (whenever you visit the tree page or it refreshes), so I would not want the extension to be pinging nbgallery with excessive frequency
• I think it needs to check if the notebook is already present, because you don't want to re-download a notebook and possibly overwrite local changes.

[cccs-cat001]

After I wrote this my org was also having concerns about notebooks that have been deleted from nbgallery that are now downloaded to jupyterhub. To incorporate all these changes I think it will need to be reworked lol