Document folders

### Desired behavior

* Users should be able to create folders for their documents.
* Users should be able to move documents into folders.
* Users should be able to browse through their folders.

(All of this very much like the Google Drive interface/functionality)

### Current behavior

* Users can only create documents and see a single list of all documents, currently.

No files, tests, or entry points are identified in the issue, so first clarify the data model and UI scope for folders, moving documents, and browsing. Done means users can create folders, move documents into them, and navigate the resulting hierarchy.

Written by the indexing model from the issue text.