Creating a way to toggle mw.config LEGACY_GLOBALS from LocalSettings (bug 28916).

Creating a way to toggle mw.config LEGACY_GLOBALS from LocalSettings (bug 28916).
* I thought a while for a way to somehow get that global variable from php to the start of the main mediaWiki object creation. Considered using a (temporary) global variable and deleting afterwards, but that looked like a hack and wasn't sure about the cross-browser functioning of it. Instead ended up by moving it to the startUp module where other global variables are accessed as well. This seems to work pretty good.

* Can be toggled from LocalSettings by setting $wgLegacyJavaScriptGlobals.
* Changed some usages of mediaWiki to use the global mw alias instead.