Use eslint for everything

eslint is much more powerful and flexible than jshint, and we don't want
to promote writing new projects with pre-ES6 code.

As a side effect, this also avoids downloading PhantomJS (see
https://github.com/jshint/jshint/issues/3318), thereby cutting down
node_modules/ from 470 MB to 210 MB.

src/app.jsx

@@ -1,4 +1,3 @@
-/*jshint esversion: 6 */
 /*
  * This file is part of Cockpit.
  *