Overhaul of Subscriptions API

Subscriptions:
- Add functionality for getting all subscribed threads, not just those with new posts
- Hide the Response class since it only is needed for GSON to stick the forum threads in
- Fix the hasUnreadThreads check
- Complete the groupThreadsBySection function to get a list of all subscribed threads grouped by their corresponding forum sections
- Remove unnecessary try/catch in catchUp

ForumThread:
- Fix some comments that were incorrectly describing the meanings of postId and lastPostId.
- Improve function naming to better reflect the meaning of the values