Add CTAs to daily emails

Add a daily email CTA node type and reference field to attach a CTA to a
daily email.

Also update the RSS feed view to include the referenced body field
value.

modules/opd_daily_emails/tests/src/Functional/CallToActionTest.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\opd_daily_emails\Functional;
+
+use weitzman\DrupalTestTraits\ExistingSiteBase;
+
+final class CallToActionTest extends ExistingSiteBase {
+
+  public function test_saving_an_email_node_without_a_cta_will_populate_one(): void {
+    $cta = $this->createNode([
+      'type' => 'daily_email_cta',
+    ]);
+
+    $email = $this->createNode([
+      'field_daily_email_cta' => NULL,
+      'type' => 'daily_email',
+    ]);
+
+    $this->assertNotEmpty($email->get('field_daily_email_cta')->getValue());
+
+    // TODO: assert the returned text.
+  }
+
+  public function test_saving_an_email_node_with_a_cta_will_keep_the_same_cta(): void {
+    $cta = $this->createNode([
+      'type' => 'daily_email_cta',
+    ]);
+
+    $email = $this->createNode([
+      'field_daily_email_cta' => $cta,
+      'type' => 'daily_email',
+    ]);
+
+    $email->set('title', 'Updated');
+    $email->save();
+
+    $this->assertSame(
+      actual: $email->get('field_daily_email_cta')->getValue()[0]['target_id'],
+      expected: $cta->id(),
+    );
+  }
+
+}