Upgrade eslint-config-wikimedia to 0.21.0

JavaScript code and comments changes:
* Remove no-multi-str disabling.
* Document $ parameter.
* Replace .substring() with .slice().
* Change assert.ok() to assert.strictEqual() in a test.

test/jquery.uls.test.js

@@ -19,7 +19,7 @@
 	QUnit.module( 'jquery.uls' );
 
 	QUnit.test( '-- Initial check', function ( assert ) {
-		assert.ok( $.fn.uls, '$.fn.uls is defined' );
+		assert.strictEqual( typeof $.fn.uls, 'function', '$.fn.uls is a function' );
 	} );
 
 }( jQuery ) );