Update handling of mobile site CSS
Provides for getting the current enwiki mobile site CSS as defined in
MediaWiki:Mobile.css, rather than relying on the outdated version of that
page that was long ago copy-pasted into the Mobileapp extension.
This could be linked directly to the live site, but for now I've chosen to
pull it in as a bundled file for continuity with the existing approach.
Note that it still gets some special treatment as we'll most likely want to
get this from enwiki production even if we want to load the rest of the
bundled CSS from MW-Vagrant or the Beta Cluster for testing.