[Form] Introduced base ExceptionInterface
[Form] Fixed EntityChoiceList when loading objects with negative integer IDs
Merge branch '2.1'

* 2.1:
  [Console] Fix style escaping parsing
  [Console] Make style formatter matching less greedy to avoid having to escape when not needed
  [Bundle] [FrameworkBundle] fixed indentation in esi.xml services file.
  [Component] [Security] fixed PSR-2 coding violation in ClassUtilsTest class.
  [Form] Fixed EntityChoiceList when loading objects with negative integer IDs
  [TwigBundle] There is no CSS visibility of display, should be visible instead
  [Form] corrected source node for a Danish translation
  [DependencyInjection] fixed a bug where the strict flag on references were lost (closes #6607)
  [HttpFoundation] Check if required shell functions for `FileBinaryMimeTypeGuesser` are not disabled
  [CssSelector] added css selector with empty string
  [HttpFoundation] Docblock for Request::isXmlHttpRequest() now points to Wikipedia
  [DependencyInjection] refactored code to avoid logic duplication
  [Form] Deleted references in FormBuilder::getFormConfig() to improve performance
  [HttpFoundation] Update docblock for non-working method

Conflicts:
	src/Symfony/Bundle/TwigBundle/Resources/views/Exception/trace.html.twig
	src/Symfony/Bundle/TwigBundle/Resources/views/Exception/traces.html.twig