Disable upload from URL on Special:ImportTranslations by default

In bug 40341, Chris Steipp identified unknown potential attack vectors in
allowing users to request arbitrary URLs via MediaWiki. This patch disables
the existing feature by adding global $wgTranslateAllowImportFromUrl. It can
be reenabled by setting $wgTranslateAllowImportFromUrl to true.

Bug: 40341
Change-Id: I01057787abcf9cbcf9796cac7750134858ebff7b