Implement restoration for StatsViewController

The WPStatsViewController class already implemented view controller
restoration, but the subclass didn't.

When a StatsViewController was restored, `self.blog` was `nil`, which
would cause it to prompt to install Jetpack, as `[nil hasJetpack]`
evaluates to false.

Fixes #1958
Yep, ugly. Copied that from `WPStatsViewController` and missed it