Revert "Revert "Add 'uk_birthday' validator that accepts DD/MM/YYYY form...

Reverts DoSomething/dosomething#3085, which in turn reverted DoSomething/dosomething#3084. 

[So basically...](http://inception.davepedu.com/noflash.php)

For review: @sergii-tkachenko 
Maybe it'd better simplify this block? It's difficult to comprehend it at once from the first sight.
Since Drupal guarantees that `Drupal.settings` it's possible to refactor it like this:

```js
var format = "MM/DD/YYYY";
if (Drupal.settings.dsValidation != null && Drupal.settings.dsValidation.dateFormat != null) {
  format = Drupal.settings.dsValidation.dateFormat;
}
```