Fix dependencies on AppDelegate navigationController

[pivotal-rebecca]

While removing extra properties that aren't used/set anymore in WordPressAppDelegate for #609, it's evident that other files are expecting a navigation controller.

EG:

https://github.com/wordpress-mobile/WordPress-iOS/blob/develop/WordPress/Classes/WPImageViewController.m#L31

Related: #603

Changing some of these to readerPostsViewController from the AppDelegate will probably work, but perhaps not all.

cc @mikejohnstn

[mikejohnstn]

Thanks Rebecca, I caught a bunch of these but suspected there were others. This is a fine checklist item to add to #603.

[pivotal-rebecca]

Assuming this is closed now from 4d22195