MAINT: Use `Type` instead of `DottedObjectName` for managers.

`Type` is strictly more powerful than `DottedObjectName` and is easier
for users to override.
MAINT: Use `Type` instead of `DottedObjectName` Log{in,out}Handler.
MAINT: Remove unused imports.

I was in the neighborhood...
DEV: Set `klass=object` for Log{in,out}Handler.

It seems plausible that people will implement custom handlers that
aren't subclasses of a known type.

Another possible option would be `IPythonHandler`, but even that feels
too restrictive.
DEV: Tweaks to Login/LogoutHandler setup.

- Require handlers to be tornado.web.RequestHandlers.  Rename the
- NotebookApp attributes from log{in,out}_handler to log{in,out}_handler_class.
Merge pull request #7254 from quantopian/type-not-dotted-object

MAINT: Use `Type` instead of `DottedObjectName` for managers.