Provide support for folders or folder "views"

[siznax]

Folders may provide an useful organization layer. Any entry or tag could be allowed to have a folder assigned. Available views would be allowed in any folder.

/folders
/             => /f:<folder>
/list         => /f:<folder>/list
/tags         => /f:<folder>/tags
/tagged/<tag> => /f:<folder>/tagged/<tag>
/notag        => /f:<folder>/notag
/search       => /f:<folder>/search
/media        => /f:<folder>/media
/media/<type> => /f:<folder>/media/<type>