Put new compact language links Beta Feature behind a feature flag

Add new $wgULSCompactLinks feature flag, defaulting to false (as is standard
for Beta Features). Only allow enabling of the feature if ULS position is
'interlanguage'.

Change-Id: Ia268c3a49b5aa14b6a00e33c7f01a61eba48e776

UniversalLanguageSelector.hooks.php

@@ -63,7 +63,9 @@ class UniversalLanguageSelectorHooks {
 		$out->addModules( 'ext.uls.init' );
 
 		// If compact ULS beta feature is enabled
-		if ( class_exists( 'BetaFeatures' ) &&
+		if ( $wgULSCompactLinks &&
+			$wgULSPosition === 'interlanguage' &&
+			class_exists( 'BetaFeatures' ) &&
 			BetaFeatures::isFeatureEnabled( $out->getUser(), 'uls-compact-links' )
 		) {
 			$out->addModules( 'ext.uls.compactlinks' );