consistent cookie-related names

- rename every cookie variable or method to 'cookies_*'
- simplify '.session.cookies' to just '.cookies'
- more consistent 'login()' structure

gallery_dl/extractor/mastodon.py

@@ -19,7 +19,7 @@ class MastodonExtractor(BaseExtractor):
     directory_fmt = ("mastodon", "{instance}", "{account[username]}")
     filename_fmt = "{category}_{id}_{media[id]}.{extension}"
     archive_fmt = "{media[id]}"
-    cookiedomain = None
+    cookies_domain = None
 
     def __init__(self, match):
         BaseExtractor.__init__(self, match)