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
I'm not sure I see what you are doing here. Split into path and file, so alike [os.path.split](https://docs.python.org/2/library/os.path.html#os.path.split) ? (might want to check behavior on windows though)