[gelbooru] use manual extraction

... to compensate for their disabled API.
(https://gelbooru.com/index.php?page=forum&s=view&id=3875)

This also adds an extractor for image-pools.

gallery_dl/extractor/deviantart.py

@@ -301,7 +301,7 @@ class DeviantartCollectionExtractor(DeviantartExtractor):
     test = [(("https://pencilshadings.deviantart.com"
               "/favourites/70595441/3D-Favorites"), {
         "url": "36ea299132a6b0a0cd319318e9bf18ad32e9b8cc",
-        "keyword": "6aada4e3159ca09ff8de9bae880952ce3b46d529",
+        "keyword": "e32b1840f1e09d1671bdc0f8a7a72472a0df57a8",
         "options": (("original", False),),
     })]