Add support for newest ForumChannel changes

This adds the following:

- Forum tag support
- Default reaction support
- Default slowmode for newly created threads

discord/abc.py

@@ -418,6 +418,11 @@ class GuildChannel:
         except KeyError:
             pass
 
+        try:
+            options['default_thread_rate_limit_per_user'] = options.pop('default_thread_slowmode_delay')
+        except KeyError:
+            pass
+
         try:
             rtc_region = options.pop('rtc_region')
         except KeyError: