[Form] Allow lenient parsing of inputs in form types date, birthday and dates in datetime

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #3730
| License       | MIT
| Doc PR        | (not yet)

This PR is about enabling lenient date parsing (name of the option debatable).
If you enable it, you can enter dates like "35.12.2013" which are automatically transformed to "4.1.2014" (this transformation happens before the validators are applied).

This was possible in earlier versions but disabled in 7e3213c.

BC is preserved by setting the default value to the current value, the only BC issue is when somebody created a form extension which added a field with the same name (unlikely).