Add shield-page callback page to the module

This is the default callback page where we redirect the user to if it does not have access. So this task needs two things
1. An Event Subscriber that will take care that whether to redirect the user based on the conditions.
2. A page where user will be redirected to.

Do you need more clarity on it. Feel free to reach on IRC : naveenvalecha

Thanks for the efforts

1. +++ b/src/EventSubscriber/ShieldPageSubscriber.php
   @@ -0,0 +1,59 @@
   +    if (\Drupal::currentUser()->hasPermission('bypass shield pages protection')) {
   

   Inject the Current user service here.

2. +++ b/src/EventSubscriber/ShieldPageSubscriber.php
   @@ -0,0 +1,59 @@
   +    $shield_pages = \Drupal::entityQuery('shield_page')
   

   Same as bove. inject the service here.

3. +++ b/src/EventSubscriber/ShieldPageSubscriber.php
   @@ -0,0 +1,59 @@
   +      ->condition('path', \Drupal::service('path.current')->getPath())
   

   same here, inject the service here.

4. +++ b/src/EventSubscriber/ShieldPageSubscriber.php
   @@ -0,0 +1,59 @@
   +      if (isset($_SESSION['_shield_pages'][$shield_page])) {
   

   instead of $_SESSION, use the request object to get and set in the session.
   e.g.
   see here https://www.drupal.org/node/2380327

Setting it to N/W
I have tested it thorougly and its not working
Testing Steps :

1. Enable the module.
2. Go to the configuration page(admin/config/system/shield-pages/settings) and change the Shield page settings to per page settings See in screenshot below.
3. Go to the Shield listing page(admin/config/system/shield-pages)
4. Add a new shield page for node 1 See the configuration in atached screen below.
5. Grant the permission 'Access password screen page' to anonymous user.
6. Now access the node/1 page using anonymous user and boom! its accessible to the user.

Added to my todo list