Add dummy form preview page

[MAILPOET-2743]

lib/Router/Endpoints/FormPreview.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace MailPoet\Router\Endpoints;
+
+use MailPoet\Config\AccessControl;
+use MailPoet\WP\Functions as WPFunctions;
+
+class FormPreview {
+  const ENDPOINT = 'form_preview';
+  const ACTION_VIEW = 'view';
+
+  /** @var WPFunctions  */
+  private $wp;
+
+  /** @var array|null */
+  private $data;
+
+  public $allowedActions = [self::ACTION_VIEW];
+  public $permissions = [
+    'global' => AccessControl::NO_ACCESS_RESTRICTION,
+  ];
+
+  public function __construct(
+    WPFunctions $wp
+  ) {
+    $this->wp = $wp;
+  }
+
+  public function view(array $data) {
+    $this->data = $data;
+    $this->wp->addFilter('the_content', [$this,'renderContent'], 10);
+    $this->wp->addFilter('the_title', [$this,'renderTitle'], 10);
+    $this->wp->addFilter('show_admin_bar', function () {
+      return false;
+    });
+  }
+
+  public function renderContent(): string {
+    return '<h1>Todo render form</h1>';
+  }
+
+  public function renderTitle(): string {
+    return __('Sample page to preview your form', 'mailpoet');
+  }
+}