EU Cookie Law banner widget breaks when more than one has been added

#### Steps to reproduce the issue

1. Open *Appearance* -> *Widgets* in wp-admin.
2. Add more than one EU Cookie Law widget, preferably in a random order.
3. Open the site and click the *Close and accept* button.

#### What I expected

The widget should fade out and the `eucookielaw` cookie should be set.

#### What happened instead

The form is submitted via a post method, instead of a `event.preventDefault` happening.

#### Suggestions

Perhaps `eu-cookie-law.js` could be mended around this, by handling an array of EU Cookie Law widgets.

Furthermore, the widget uses an `id` attribute that's not unique. Depending on a CSS class would be a better way to handle multiple copies of the same widget.

Start with eu-cookie-law.js and reproduce the issue by adding multiple EU Cookie Law widgets in Appearance → Widgets, preferably in random order. Trace how the close-and-accept action selects widgets and handles submission; done means each widget fades out without a POST and the eucookielaw cookie is set.

Written by the indexing model from the issue text.