Custom #infinite-handle button doesn't works

Custom `#infinite-handle` button doesn't works because there is incorrect `click` event handler: https://github.com/Automattic/jetpack/blob/3fe20501f562c5a8c8009d43e95b50620b957d29/modules/infinite-scroll/infinity.js#L98

#### Steps to reproduce the issue

```php
add_theme_support( 'infinite-scroll', array(
'type' => 'click',
'click_handle' => false,
) );
```

1. Create button with ID `infinite-handle` `Show more`
2. Set `'click_handle' => false,`

#### What I expected

Button works as default button.

#### What happened instead

Nothing happens.

---

The root of the issue is incorrect `this.handle` assignment to new element instead of existed one: https://github.com/Automattic/jetpack/blob/3fe20501f562c5a8c8009d43e95b50620b957d29/modules/infinite-scroll/infinity.js#L47-L49

Start in modules/infinite-scroll/infinity.js at the handle assignment around lines 47-49 and the click handler around line 98. Reproduce the issue with a custom #infinite-handle button and click_handle set to false; done means the custom button triggers the default infinite-scroll loading behavior.

Written by the indexing model from the issue text.