[Form] Implemented support for buttons
[Form] Made submit buttons able to convey validation groups
[Form] Changed isset() to array_key_exists() to be consistent with ParameterBag
[Form] Fixed concatenation operator CS (see 7c47e34928c39e4797edc841423237a16588395e)
[Form] The option "validation_groups" can now be set to false to disable validation. This is identical to setting it to an empty array.
[Form] Fixed invalid use of FormException
[Form] Fixed incorrect doc comment
[Form] Simplified Twig code
[Form] Removed deprecated code scheduled for removal in 2.3
[Form] Added leading backslashes to @exceptionMessage doc blocks