Speed up or avoid major scraping operations.

Figure out which categories, forums, posts, users, threads, and thread tags are
involved, then fetch them all at once, when scraping:
* The forum hierarchy.
* A list of threads.
* The Leper's Colony or a rap sheet.
* A list of private messages.

In addition, when checking for new private messages, use a new scraper that
quickly checks the count. We'll only bother updating/inserting
AwfulPrivateMessage objects when refreshing the Private Messages tab.

Bonus:
* Switch to -[HTMLNode textContent] (from -innerHTML) where appropriate.
* Remove private message table view controller from notification center.