Improve the discoverability of the Deepnote explorer

If you open a Deepnote file from the file explorer, you might not discover the Deepnote tab and the Deepnote explorer. 

User feedback:
> I had no idea how to actually view the various notebooks within projects. The only way I figured this out by watching someone's Loom on it.

> We should have a dedicated button added to the top. 'Project Explorer' or something similar.
