Revert "Revert "Add 'uk_birthday' validator that accepts DD/MM/YYYY format.""
Revisions from PR #3088.
Continued revisions from PR #3088.
Merge pull request #3088 from DoSomething/revert-3085-revert-3084-uk-phone-validator

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