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 
- Remove the whithespace at the end of the line
- Always put comma after every array element, see [Drupal coding standarts](https://www.drupal.org/coding-standards#array):

> Note the comma at the end of the last array element; This is not a typo! It helps prevent parsing errors if another element is placed at the end of the list later.