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 
It's not only UK format, I think it's the same for all EU countries. Let me check.