Add space between back/forward and page selector on posts view toolbar.

It was unnecessarily difficult to tap the back or forward buttons without
inadvertently pressing the page selector button.

Bonus: prefix UIBarButtonItem category methods.
Little fixes.

* Stop listening for notifications on no-longer-needed background managed
  object contexts in the Forums client.
* Unconditionally give up on fetching posts if the URL request can't be formed
  (i.e. doesn't matter if we have a completion block).
* Formatting.
Fetch 20 threads at a time when listing them.

iPad in portrait only shows 13 at a time, and I'm (this is a longshot) hoping
this will help track down a bug with the "sort bookmarks by unread posts"
setting seemingly not working.
Attempt to fix broken "sort unread threads first" setting.

Unbeknownst to me, if you make certain updates in another managed object
context and use -mergeChangesFromContextDidSaveNotification: to bring the
changes into the main context, a fetched results controller may not react if
the updated objects were faults. To work around this issue, we fire all faults
before merging in the changes.

I have a hard time reproducing the bookmark sorting bug, so I'm not sure this
fixes it. But it seems like a thing we need to do regardless.
Swipe to delete bookmarks.

Took this out awhile ago when (pointlessly) redoing things, and never put it
back in. Until now.
One action sheet popup at a time please.

Awful Browser and the image preview would pop up another action sheet on iPad
each time one tapped the action item, regardless of the presence of an action
sheet.

Bonus: formatting.
Fix previewing attached images and following relative links in posts.

With the awful-resource:// change we set the base URL of the web view
appropriately, but that means the URLs we pull from the page may be relative.
Fix state restoration for Awful Browser.

We hadn't set a restorationClass and the app delegate couldn't find it, so we'd
restore the navigation controller but not the actual browser view controller,
leaving people with a black screen and an itemless navigation bar.

In fixing this (by setting ourselves as the restoration class), we're now
saving the restoring the UIWebView's state.
Remove pointless back bar setting in posts view controller.

It's always empty, no need to set it again.