Fix crash when toggling favorites.

The flawed hit test would cause hidden cells' favorite buttons to fire. Hidden
cells don't have a related forum, and when we'd go to insert the nil forum into
an array, we'd crash.
Clarify section munging.
Check for parsing errors when locating a post.
Formatting and tiny cleanup.
Fix crash loading a new page of posts after scrolling.

We cache post updates while scrolling the posts view so that scrolling doesn't
get all jittery. Unfortunately, by not cleaning out this cache, it was possible
to run the cached updates after we'd moved on to the next page. This can crash
when, for example, a cached update would reload a post index beyond the next
page's number of posts.

The fix is to simply delete the update cache if we're loading a new page.