Add opt-out setting for Gravatar integration

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

lib/Service/Avatar/GravatarSource.php

@@ -38,6 +38,15 @@ class GravatarSource implements IAvatarSource {
 		$this->clientService = $clientService;
 	}
 
+	/**
+	 * Does this source query external services?
+	 *
+	 * @return bool
+	 */
+	public function isExternal() {
+		return true;
+	}
+
 	/**
 	 * @param string $email sender email address
 	 * @param AvatarFactory $factory