Reader: Hide edit button when user is not subscribed to any tags

Fixes #2365
This might be a little nitpicky. There is an `NSNotification` dispatched when the selected topic is changed. You might consider tweaking the name of both the custom type, and the property name to make it clear that this isn't related to the selected topic. What do you think about `TopicListChanged topicListChangedBlock;` or something similar.