Add support for enabling and displaying global interwikis

This effectively merges Extension:ShoutWikiInterwikiMagic into Interwiki
to add support for global interwikis, allowing sysadmins to set
$wgInterwikiCentralDB to a central wiki database from which to pull global
interwikis, which will be used alongside any locally-defined prefixes.

It also separates out the display of global, local, and language
interwikis into their own sections, improving the display even for
local-only projects.

Bug: 68241
Change-Id: I54c072c65f10dd0ad5f815c2649e7ce9ed1e7b58