Fixing neglect to determine whether a user has the permission to create ...

...a topic on a category (besides being able to create a post) in ListController, TopicList, and TopicListSerializer causing the "Create Topic" button to appear even if a user cannot actually create a topic in that category but can reply to a topic therein.

Updated from [1659](https://github.com/discourse/discourse/pull/1659) to resolve a merge conflict created by @eviltrout cleanly. Final update.