Apply strict mode for groonga-admin.js in admin page.

Today, we can use [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode) of JavaScript (ECMAScript).

Strict mode makes opaque behaviors in JavaScript more clearly, and warns some early errors to increase debug-ability. So I think this pull request makes the ready to introduce more modern/easy development in this web ui.