Download real magic-folder contents

• Add a App abstraction for the global application state, including magic-folders and their contents
• Add a dependency on magic-foldr and use it to download magic-folder contents when a folder is first visited
• Use the App value to save previously downloaded content so it doesn't have to be re-downloaded on every render

As with the grid config, the magic-folder config is hard-coded to something that is only sensible on my local network.

This is stacked on !51 (merged)