[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
[Form] Improved test coverage of widget_attributes and widget_container_attributes blocks
feature#9171 [Form] Rewrite boolean attributes to match HTML spec (bschussek)

This PR was merged into the master branch.

Discussion
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[Form] Rewrite boolean attributes to match HTML spec

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Same as #7856

> '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.

Commits
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b85577b [Form] Improved test coverage of widget_attributes and widget_container_attributes blocks
8e4c2a7 [Form] Rewrite boolean attributes to match HTML spec