[Form] Added additional data attributes for collections

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | na
| License       | MIT
| Doc PR        | WIP

- [x] fix the tests as they have not been updated yet
- [ ] submit changes to the documentation

----

I've added 4 extra data-* attributes to collections in twig and php views for easier js manipulation.

New attributes:

* `data-form-widget='collection'`: This is a collection and all children are form types
* `data-collection-allow-add`: Allow adding of new elements
* `data-collection-allow-delete`: Allow deletion of elements (new and existing)
* `data-prototype-name`: The prototype name

This should make javascript integration much easier.

Example js:

```js
$('[data-form-widget=collection]').each(function(){

    var $collection = $(this),
        prototypeString = $collection.data('prototype'),
        prototypeName = $collection.data('prototypeName') || '__name__',
        prototypeNameRegx = new RegExp(prototypeName+"(label__)?","g"),
        count = $collection.children().length;

    var $deleteLink = $("<span />", {
        'class': 'js-delete',
        'text': 'Delete'
    });

    var $addLink = $("<span />", {
        'class': 'js-add',
        'text': 'Add'
    });

    if($collection.data('collectionAllowDelete')) {

        $collection
            .children()
            .append($deleteLink.clone(true));

        $collection.on('click', '.js-delete', function(){
            event.preventDefault();
            $(this).parent().remove();
        });
    }

    if($collection.data('collectionAllowAdd')) {

        $addLink.click(function(event){
            event.preventDefault();
            var newPrototypeString = prototypeString.replace(prototypeNameRegx, count),
                $newPrototype = $(newPrototypeString).append($deleteLink.clone(true));
            $newPrototype.insertBefore($addLink);
            count++;
        });

        $collection.append($addLink);
    }
});
```

Replaces #5145