Allow two-tier setup of transwiki import sources

Allow two-tier setup of transwiki import sources

There has been some demand, particularly in the Wikimedia cluster, for the
ability to import from any wiki of a cluster. For this to occur, the
transwiki import user interface needs a bit of a rethink.

This patch replaces the existing single dropdown with a pair of dropdowns:
the first to select the wiki project, and the second to select the
subproject (e.g. a Wikipedia/Wiktionary language, or a Wikia site).
The second one is optional (to support single-wiki sites like Meta, or
for backwards compatibility with existing setups).

$wgImportSources is now treated as a mixed array/associated array (see
comment in DefaultSettings.php). Existing configurations will still work
but will receive no new functionality.

The non-JavaScript fallback is not pretty, but (a) it works, (b) I don't
see an easy way to make it nicer, and (c) wiki sysops should probably be
using a JavaScript-enabled browser for admin actions like importing...

The intention is to alter the WMF configuration to automatically populate
$wgImportSources with all public cluster wikis. I'm not exactly sure how
this will be set up, but this patch is an important first step. I expect
some non-WMF users of MediaWiki will find it helpful as well.

Change-Id: Icdb655500c1ae5374dc7a9f4d99e6738b2269b14