Allow non-standard devise user class names

[cllns]

I have my User class in a namespaced Engine called Accounts, so I need to override the belongs_to association as class_name: "Accounts::User" in a redactor.rb file.

The current configuration allows for differently named User classes, via foreign-key inference, but doesn't allow for namespaced User classes. This commit adds support for namespaced User models.

In my app, I first monkey patched this code change, and now I'm using my branch in my Gemfile.