Set $wgIncludeLegacyJavaScript false by default

Set $wgIncludeLegacyJavaScript false by default

This setting was deprecated in 0ac4f998 but was left as true by default, which
means sysadmins weren't prompted to find and fix faulty code. Setting this to
false by default gives them a release in which to fix the code (setting this
back to true until such fixes are made) before we remove it entirely in the
next MediaWiki release. Extensions, skins, gadgets and scripts that need the
mediawiki.legacy.wikibits module should express a dependency on it in their
ResourceLoader manifest.

Bug: T35836
Change-Id: If787d0975b6019827e81f4ea0d90898e361d9acd