Use system language as default wiki search language

This will set the search language to the system locale lang
unless you've set an explicit setting -- previously we simply
used English as a default, whoops!

Also loads the appropriate Main Page on first view (note if
switching languages after first run, we'll show whatever was
shown previously, even if it's a main page).

Moved a few bits from SessionSingleton and WebViewController
to WikipediaAppUtils:
* domainNameForCode
* mainArticleTitleForCode
* copyAssetsFoldertoAppDataDocuments

also added:
* wikiLangForSystemLang

Currently the system->wiki langugage mappings simply throw away
any variant or country code and take the rest as-is; should
probably do a quick survey to make sure there aren't any
conflicts with any of the languages supported by iOS.

Change-Id: Iff978197949940ec53bb4c13bf5eb71f4dc0ba7f