AuthenticatedFileHandler uses ContentsManager to serve files and notebooks

In reference to #6613 I'm adding this PR. Rather than working on paths, it will use the current ContentsManager to get the contents of a file.

I tested this with the default `FileContentsManager` and our own `MongoContentsManager`.

There might be a better way to serve the file instead of `write()` and `flush()`.

Let me know what you think
> Where can I put my own FilesHandler? How to switch it later?

My initial thought is that it would be `FilesHandler` in `services/contents/handlers`. I don't know what you mean by 'switch it later' though.