Make Authentication more configurable

[peakpg]

Future Ideas:

• Configure which section the /login and /forgot-password are considered in for purposes of menus.
• Provide a way to configure redirection after logging in.
• Allow developers to configure what data is returned by /cms/user.json when its called.
• Provide a way to use different versions of JQuery on public sites, or disable jquery if developers want it. (AJAX is used for 'Hello User' messaging currently)

[peakpg]

Allow Cms::ExternalUser to be easily subclassed by updating the .authenticate method like so:

def self.authenticate(login, source, info={})
      info = work_around_rails_4_serialization_bug(info)
      criteria = {login: login, source: source}
      existing = where(criteria).first
      if existing
        existing.update(info)
        return existing
      end
      criteria.merge!(info)
      new_user = self.create!(criteria)
      new_user.groups << Cms::Group.guest if Cms::Group.guest
      new_user
    end