[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 without a 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.