Allow folders in `/assets/`

Hi,

It seems that right now, the assets folders can only have 1 level. Ie, if you create a folder like assets/foo and put an asset in it (assets/foo/bar.png), then it won't be listed when you use the assets search, and you can't link to it.

I think it would be nice if we could organize assets with folders, and they would show in the assets list.

Thanks!