New editor Navbar fixes

Fixes https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/199 and https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/264

Rejiggered the ```WPBlogSelectorButton``` so it has two modes (stacked and single line). This button should now update properly after a new blog is selected. 

Also adjusted the layout of the navbar on the new editor a little bit.

/cc @diegoreymendez and @aerych (Since I touched the ```RebloggingViewController```)

Just wondering if it'd make sense to avoid recreating the button when this method is called.