Reset posts view on iPad when logging out.

It used to show the last thread you were reading behind the login form.

The notification stuff is to deal with the posts view's attempt to mark posts
as read when it disappears.
instancetype is cool
Fix logging in and use proper error codes.

The SA Forums recently changed how logging in works in a way that affects how
we check for success or failure.

We were also using made up error codes in the NSCocoaErrorDomain instead of
making our own proper AwfulErrorDomain. That was dumb and is now fixed.
Note for handier logging in in the future.