FIX: Store user's id instead for sending activation email.

* Email and username are both allowed to be used for logging in.
  Therefore, it is easier to just store the user's id rather than
  to store the username and email in the session.

app/controllers/session_controller.rb

@@ -278,7 +278,7 @@ class SessionController < ApplicationController
   end
 
   def not_activated(user)
-    session[ACTIVATE_USER_KEY] = user.username
+    session[ACTIVATE_USER_KEY] = user.id
     render json: {
       error: I18n.t("login.not_activated"),
       reason: 'not_activated',