[RFC][WIP][Form] Rewrite boolean attributes to match HTML spec

> 'The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.' - http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#boolean-attribute

This commit modifies widget_container_attributes and widget_attributes so that:

* `true` values render as the attribute name with the attribute name repeated as the value
* `false` values are not rendered

The comparison is strict using sames() in twig.

Previously `false` values would have been rendered as `some-attribute=""` which according to the spec would actually make them a boolean attribute and therefore equal to true.

Todo:

- [ ] Write tests
- [ ] Update docs
- [ ] Update PHP templates