Treat folders as first-class citizens

Right now folders are implemented using tags, which has led to their existence being dependent on them having list children. This means you can't have empty folders, or modify folders directly (instead of renaming them, you have to move all its contents into a new folder with the right name).

We'll probably need new web services for folders to improve on what we have now.

Start by tracing how folders are currently represented through tags and how their list children are exposed. Then inspect the existing web-service layer to determine the required folder operations. Done means empty folders can exist and folders can be modified directly, including renaming without moving their contents.

Written by the indexing model from the issue text.