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 
Yes, check this out: http://en.wikipedia.org/wiki/Date_format_by_country
I think we better name it something like `forceDateFormat` and set it to `DMY` or `d/m/Y`