[FEATURE] Enable subject to contain markers

Add the marker replacement to the subject, so markers can be used
inside the subject, too

Classes/Service/MailTemplateService.php

@@ -325,19 +325,23 @@ class MailTemplateService {
 
 		/** @var StandaloneView $emailView */
 		$emailView = $this->objectManager->get(StandaloneView::class);
-
+		$emailView->assignMultiple($this->markers);
 		if (NULL === $defaultTemplateContent) {
+			$emailView->setTemplateSource(\trim($template->getSubject()));
+			$subject = $emailView->render();
+
 			$emailView->setTemplateSource($template->getContent());
-			$subject = trim($template->getSubject());
 		} else {
-			$emailView->setTemplateSource($defaultTemplateContent);
-
 			$subject = self::getRegisterArray()[$this->extensionKey][$this->templateName]['subject'];
-			if (is_array($subject)) {
-				$subject = trim(
+			if (\is_array($subject)) {
+				$subject = \trim(
 					self::getRegisterArray()[$this->extensionKey][$this->templateName]['subject'][$this->language]
 				);
 			}
+			$emailView->setTemplateSource($subject);
+			$subject = $emailView->render();
+
+			$emailView->setTemplateSource($defaultTemplateContent);
 		}
 
 		if ($this->subject !== '' && $this->subject !== NULL) {
@@ -346,7 +350,6 @@ class MailTemplateService {
 
 		$this->mailMessage->setSubject($subject);
 
-		$emailView->assignMultiple($this->markers);
 		$emailBody = $emailView->render();
 
 		// insert <br /> tags, but replace every instance of three or more successive breaks with just two.