Fix the 404 message for product alerts when not logged in

When the user isn't logged in and presses the product alert link
(either stock or price), the user has to login and gets redirected
to a 404 page. This is because the redirect done by the login page
is a `GET` call. The fix done is to change the used interface for
the product alerts from `POST` to `GET`, as this is the easy fix and
there's no reason to use `POST` here at this moment, as all parameters
for the call are all in get URL parameters.

Author: Arjen Miedema

app/code/Magento/ProductAlert/Controller/Add/Price.php

@@ -6,7 +6,7 @@
 
 namespace Magento\ProductAlert\Controller\Add;
 
-use Magento\Framework\App\Action\HttpPostActionInterface;
+use Magento\Framework\App\Action\HttpGetActionInterface;
 use Magento\ProductAlert\Controller\Add as AddController;
 use Magento\Framework\App\Action\Context;
 use Magento\Customer\Model\Session as CustomerSession;
@@ -20,7 +20,7 @@
 /**
  * Controller for notifying about price.
  */
-class Price extends AddController implements HttpPostActionInterface
+class Price extends AddController implements HttpGetActionInterface
 {
     /**
      * @var \Magento\Store\Model\StoreManagerInterface

app/code/Magento/ProductAlert/Controller/Add/Stock.php

@@ -6,7 +6,7 @@
 
 namespace Magento\ProductAlert\Controller\Add;
 
-use Magento\Framework\App\Action\HttpPostActionInterface;
+use Magento\Framework\App\Action\HttpGetActionInterface;
 use Magento\ProductAlert\Controller\Add as AddController;
 use Magento\Framework\App\Action\Context;
 use Magento\Customer\Model\Session as CustomerSession;
@@ -19,7 +19,7 @@
 /**
  * Controller for notifying about stock.
  */
-class Stock extends AddController implements HttpPostActionInterface
+class Stock extends AddController implements HttpGetActionInterface
 {
     /**
      * @var \Magento\Catalog\Api\ProductRepositoryInterface

app/code/Magento/ProductAlert/view/frontend/templates/product/view.phtml

@@ -6,7 +6,7 @@
 ?>
 <?php /* @var $block \Magento\ProductAlert\Block\Product\View */?>
 <div class="product alert <?= $block->getHtmlClass() ?>">
-    <a href="#" data-post='<?= /* @noEscape */ $block->getPostAction() ?>'
+    <a href="<?= $block->escapeUrl($block->getSignupUrl()) ?>"
        title="<?= $block->escapeHtml(__($block->getSignupLabel())) ?>" class="action alert">
         <?= $block->escapeHtml(__($block->getSignupLabel())) ?>
     </a>