Export plain GridFS via WebDAV

[thiloplanz]

v7files is using its own storage system, which is similar but not compatible with GridFS. Some people may prefer to use v7files as a simple WebDAV server for their existing GridFS content. There should be an option to do that.

It would use GridFS directly, so there would be no content compression, de-duplication, or revision history.

It should still be possible to achieve a folder structure and permission settings with small additions to the standard. These additions should be optional and must not conflict with the ability for other GridFS client software to interact with the files directly (which persumably was the point to want to use GridFS).