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
Thanks for the useful input. I'll correct those points until tomorrow. I'm just not certain about the write/flush and the return value.