Support hashed password

[jluttine]

The Dashboard basic authentication uses username and password. However, the password is provided in plain text. For improved security, I'd prefer providing it as a salted SHA hash as in Jupyter Notebook authentication: http://jupyter-notebook.readthedocs.io/en/latest/public_server.html#preparing-a-hashed-password. Would it be possible to support or is there some (simple) way to do this already?