Automatically show a default "Public" tree to non-authenticated users

[bmann]

Currently, we have a splash page on the Preview of Drive which explains a bit what it is and has a default IPFS hash for boris.fission.name that you can explore after you click explore.

Rather than having that splash page, we should display a "Public" tree by default. We'll populate this with content that is appropriately licensed for sharing, and when people sign up for a Fission account and are authenticated, this remains in their Public folder.

See https://bmann.fission.name/Drive/public for the current snapshot. This hash would be "hardcoded" into Drive (with a full Fission File System, so not exactly this version), and when we create an FFS for a new user, again this hash would be grafted into the Public tree.

This means an anonymous person can browse and preview a public tree without an account.

We also want a "web banner" that displays in anonymous mode, which explains what these public files are, and directs them to create an account (which links to auth).