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.

package.json

@@ -25,7 +25,7 @@
 		"Siebrand Mazeland"
 	],
 	"devDependencies": {
-		"eslint-config-wikimedia": "0.20.0",
+		"eslint-config-wikimedia": "0.21.0",
 		"eslint-plugin-html": "6.1.2",
 		"grunt": "1.4",
 		"grunt-contrib-qunit": "4.0.0",