Added iPad support

With the Cartography-based layout, everything Just Works™ on iPad.  There’s no reason why this shouldn’t be a universal app.

• Switched the .xcodeproj’s Deployment Info › Devices from “iPhone” to “Universal”.
• Created a new `LaunchImage-iPadRetina.png` (based on `LaunchImage-iPhone6Plus.png`) and added it to the `LaunchImage.launchimage` asset catalog.
• Updated `UIImage+LaunchImage.swift` to choose the new image when the screen’s retina-ignored height is 1024 (so, all iPads).
	• Minor: Added a note based on my findings and Stack Overflow about how the default case won’t actually “Let the system decide”— it just won’t work.