treeview update style and init script

**Note**

- Most standard, reusable treeview javascript script (possible to replace the one used for the menu by this one).
- New styles for this dynamic treeview added to the current one.

**Toggleable tree markup (inspired by PageAdmin/tree.html.twig) :**

```html
<ul class="page-tree page-tree--toggleable js-treeview">
    <li>
        <div class="page-tree__item">
            <i class="fa fa-caret-right" data-treeview-toggler></i>
            <a class="page-tree__item__edit" href="/">Rubrique A</a>
        </div>
        <ul>
            <li>
                <div class="page-tree__item is-active">
                    <i class="fa fa-caret-right" data-treeview-toggler></i>
                    <a class="page-tree__item__edit" href="/">Sous-rubrique AA</a>
                </div>
                <ul>
                    <li>
                        <div class="page-tree__item">
                            <a class="page-tree__item__edit" href="/">Sous-rubrique AAA</a>
                        </div>
                    </li>
                </ul>
            </li>
        </ul>
    </li>
    <li>
        <div class="page-tree__item">
            <i class="fa fa-caret-right" data-treeview-toggler></i>
            <a class="page-tree__item__edit" href="/">Rubrique B</a>
        </div>
        <ul>
            <li>
                <div class="page-tree__item">
                    <a class="page-tree__item__edit" href="/">Sous-rubrique BB</a>
                </div>
            </li>
        </ul>
    </li>
</ul>
```