Adds the ability to export an album using streaming tar.

controllers/ViewPhotoAlbum.php

@@ -61,7 +61,13 @@ class ViewPhotoAlbum extends HTMLController
 		// Can we do fancy things here?
 		// !!! TODO: permission system?
 		$buttons = [];
+
 		if (Registry::get('user')->isLoggedIn())
+			$buttons[] = [
+				'url' => BASEURL . '/download/?tag=' . $id_tag,
+				'caption' => 'Download this album',
+			];
+
 			$buttons[] = [
 				'url' => BASEURL . '/uploadmedia/?tag=' . $id_tag,
 				'caption' => 'Upload new photos here',