Fix crash loading empty PM tag images.
Fix showing empty tag images.
Stop trying to initialize images with nil image names.

Fixes the "CUICatalog: Invalid asset name supplied" logs. They ultimately come
from calling +[UIImage imageNamed:] with a nil image name.
Create and destroy root view controller stack on login.

Inspired by seeing crashes when logging out on iPad. Previously we attempted to
clear out all open view controllers when logging out. Simply tearing them all
down is conceptually much simpler.

During the implementation, I fixed retain cycles in AwfulForumsListController
and AwfulPostsViewController with a couple block callbacks.

As part of the simplification, there's no longer any need for the "will/did log
out" notifications, and AwfulURLRouter is now created and destroyed along with
the root view controller.

And, because I was there, AwfulHTTPClient's logInAs... method now scrapes the
HTML (not JSON) profile page and creates an AwfulUser object after logging in.
Formatting.
Fix looking up (some) posts from Leper's Colony.
Recreate forums as needed during state restoration.

Maybe the data store failed migration or something. We should still restore
state.

In addition, update the title after refreshing. Especially noticeable when
recreating the forum during state restoration (previously it would stay blank),
but in theory the name could've changed anyway.