Add sliding menu

The contents of ``base.js`` right now are:

```javascript

"use strict";

var $ = require('jquery'),
    csrf = require('./csrf');

// Place JS here if it needs to be executed on every page.
require('bootstrap');

$.ajaxSetup(csrf.jqueryAjaxSetupOptions);
```

Since jQuery is already ``require``d you should be able to just modify ``base.js`` to look like this:

```javascript

"use strict";

var $ = require('jquery'),
    csrf = require('./csrf');

// Place JS here if it needs to be executed on every page.
require('bootstrap');

$.ajaxSetup(csrf.jqueryAjaxSetupOptions);

// Enable the burger menu
$('.nav-menubutton').on('click', function (a) {
    $('body').toggleClass('menu-active');
});
```