Refactor UsersController#create

* Simplify controller action
* Extract service classes

app/services/user_authenticator.rb

@@ -0,0 +1,39 @@
+class UserAuthenticator
+  def initialize(user, session, authenticator_finder = Users::OmniauthCallbacksController)
+    @user = user
+    @session = session[:authentication]
+    @authenticator_finder = authenticator_finder
+  end
+
+  def start
+    if authenticated?
+      @user.active = true
+    else
+      @user.password_required!
+    end
+  end
+
+  def finish
+    if authenticator
+      authenticator.after_create_account(@user, @session)
+    end
+
+    @session = nil
+  end
+
+  private
+
+  def authenticated?
+    @session && @session[:email] == @user.email && @session[:email_valid]
+  end
+
+  def authenticator
+    if authenticator_name
+      @authenticator ||= @authenticator_finder.find_authenticator(authenticator_name)
+    end
+  end
+
+  def authenticator_name
+    @session && @session[:authenticator_name]
+  end
+end