Do not use the main config for extension configuration options

This currently just works, because the currently used backend are
global variables. Once the main config uses a more scoped/focused
backend, such as a HashConfig or a database-based config, requesting
extension configurations from it will not work anymore.

Therefore, use the extensions Config object to request configuration
options for the extension.

Change-Id: Iea02d3c284f9a0e12abff33e3624dce9585ab51c

extension.json

@@ -18,6 +18,9 @@
 	"requires": {
 		"MediaWiki": ">= 1.25.0"
 	},
+	"ConfigRegistry": {
+		"revisionslider": "GlobalVarConfig::newInstance"
+	},
 	"config": {
 		"@RevisionSliderBetaFeature": "Make the RevisionSlider feature a BetaFeature. Setting this requires the RevisionSlider extension and requires each user to enable the BetaFeature.",
 		"RevisionSliderBetaFeature": false,

src/RevisionSliderHooks.php

@@ -10,12 +10,31 @@ use MediaWiki\MediaWikiServices;
  */
 class RevisionSliderHooks {
 
+	/**
+	 * @var Config
+	 */
+	private static $config;
+
+	/**
+	 * Returns the RevisionSlider extensions config.
+	 *
+	 * @return Config
+	 */
+	private static function getConfig() {
+		if ( self::$config === null ) {
+			self::$config = MediaWikiServices::getInstance()
+				->getConfigFactory()
+				->makeConfig( 'revisionslider' );
+		}
+		return self::$config;
+	}
+
 	public static function onDiffViewHeader(
 		DifferenceEngine $diff,
 		Revision $oldRev,
 		Revision $newRev
 	) {
-		$config = MediaWikiServices::getInstance()->getMainConfig();
+		$config = self::getConfig();
 
 		/**
 		 * If this extension is configured to be a beta feature, and the BetaFeatures extension
@@ -112,7 +131,7 @@ class RevisionSliderHooks {
 	}
 
 	public static function getBetaFeaturePreferences( User $user, array &$prefs ) {
-		$config = MediaWikiServices::getInstance()->getMainConfig();
+		$config = self::getConfig();
 		$extensionAssetsPath = $config->get( 'ExtensionAssetsPath' );
 
 		if ( $config->get( 'RevisionSliderBetaFeature' ) ) {