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/pornhub.py

@@ -58,7 +58,7 @@ class PornhubGalleryExtractor(PornhubExtractor):
         self._first = None
 
     def items(self):
-        self.session.cookies.set(
+        self.cookies.set(
             "accessAgeDisclaimerPH", "1", domain=".pornhub.com")
 
         data = self.metadata()