remove handlebars and use underscore templates for address finder

I think it would be better to move these templates in Jinja templates. This was it will be easier to translate.

```html
{% block javascripts %}
  {{ super() }}

  <script type="text/html" id="addressFinderButton">
    <button class="button-secondary address-finder-button" type="button">
      Find UK address
    </button>
  </script>
{% endblock %}
```

and in JS

```js
var button = $('#addressFinderButton').html();
this.$postcode.after(button);
```

Or something along those lines.