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/Module/Photo.php

@@ -22,6 +22,7 @@
 namespace Friendica\Module;
 
 use Friendica\BaseModule;
+use Friendica\Contact\Header;
 use Friendica\Core\Logger;
 use Friendica\Core\Protocol;
 use Friendica\Database\DBA;
@@ -482,6 +483,6 @@ class Photo extends BaseApi
 		if (!empty($photo)) {
 			return $photo;
 		}
-		return MPhoto::createPhotoForImageData(file_get_contents(DI::basePath() . '/images/friendica-banner.jpg'));
+		return MPhoto::createPhotoForImageData(file_get_contents(DI::basePath() . (new Header(DI::config()))->getMastodonBannerPath()));
 	}
 }