Initial support for webfonts in MobileFrontend

Written in pair-programming with Ryan Kaldari.

This adds a version of webfonts initialization that is stripped
of any smart features. It only loads the basic jquery.webfonts
library and the fonts repository in a way that is usable
with the MobileFronted extension.

It is completely disabled by default.

It can be enabled by setting
$wgULSMobileWebfontsEnabled = true;
and even then it will only be enabled for users
who opted in for the mobile beta mode.

Change-Id: I488e411232a1192bbf46ae5675c701366bda9a3e
Merge "Initial support for webfonts in MobileFrontend"