Replace remaining references to default banner image by api.mastodon_banner configuration value

- Ensure leading slash is present at every place the configuration value is used

src/Contact/Header.php

@@ -0,0 +1,47 @@
+<?php
+/**
+ * @copyright Copyright (C) 2010-2023, the Friendica project
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ */
+
+namespace Friendica\Contact;
+
+use Friendica\Core\Config\Capability\IManageConfigValues;
+
+class Header
+{
+	/** @var IManageConfigValues */
+	private $config;
+
+	public function __construct(IManageConfigValues $config)
+	{
+		$this->config = $config;
+	}
+
+	/**
+	 * Returns the Mastodon banner path relative to the Friendica folder.
+	 *
+	 * Ensures the existence of a leading slash.
+	 *
+	 * @return string
+	 */
+	public function getMastodonBannerPath(): string
+	{
+		return '/' . ltrim($this->config->get('api', 'mastodon_banner'), '/');
+	}
+}