Mobile support in MediaWiki core

Rationale
By default, MediaWiki does not offer mobile device-specific support, making MediaWiki sites very difficult to use on mobile devices. This has been mitigated in many ways by Extension:MobileFrontend, however there is much to be gained by building native mobile support into MediaWiki core. Such as:


 * Increased performance
 * A better framework for other extensions to make mobile-specific features
 * Out-of-the-box mobile support for MediaWiki sites
 * A present and future-ready device-agnostic core that can display data in a multitude of different ways, increasing MediaWiki's flexibility
 * Greater developer and end-user support by increasing exposure to mobile-specific issues