Devise logout for Cms::ExternalUser

[jamesprior]

Devise cannot find an authentication strategy when logging out a Cms::ExternalUser. I suspect it is because Cms::ExternalUser does not inherit from Cms::User, so when devise receives a Cms::ExternalUser to log out it doesn't match it to an existing strategy. I think perhaps if Devise had an authentication strategy for Cms::PersistentUser it would work but that is currently un-tested.

[jamesprior]

FYI it looks like this is an easy config fix to the devise_for command. I would like to test it a bit more before making a PR though.