Extract System emails from enotify

- Removed every SYSTEM_EMAIL occurrence in enotify
- Introduced a "SystemMailBuilder" for build system emails
- Replaced every SYSTEM_EMAIL usage in the classes with calling this builder
- Added tests for this new Builder
- Split the email templates between "base" template for email and concrete usages for different use cases

tests/Util/SampleMailBuilder.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace Friendica\Test\Util;
+
+use Friendica\Util\EMailer\MailBuilder;
+
+class SampleMailBuilder extends MailBuilder
+{
+	/** @var string */
+	protected $subject;
+	/** @var string */
+	protected $html;
+	/** @var string */
+	protected $text;
+
+	/**
+	 * Adds a test message
+	 *
+	 * @param string $subject The subject of the email
+	 * @param string $html    The preamble of the email
+	 * @param string $text    The body of the email (if not set, the preamble will get used as body)
+	 *
+	 * @return static
+	 */
+	public function withMessage(string $subject, string $html, string $text)
+	{
+		$this->subject = $subject;
+		$this->html    = $html;
+		$this->text    = $text;
+
+		return $this;
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	protected function getSubject()
+	{
+		return $this->subject;
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	protected function getHtmlMessage()
+	{
+		return $this->html;
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	protected function getPlaintextMessage()
+	{
+		return $this->text;
+	}
+}