[danbooru] inherit from BaseExtractor

- merge danbooru and e621 code
- support booru.allthefallen.moe (closes #2283)
- remove support for old e621 tag search URLs

gallery_dl/extractor/__init__.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2015-2021 Mike Fährmann
+# Copyright 2015-2022 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -31,7 +31,6 @@ modules = [
     "desktopography",
     "deviantart",
     "dynastyscans",
-    "e621",
     "erome",
     "exhentai",
     "fallenangels",