Fixes #1412: `FileValidator` still triggers `uploadRequired` error in some case when `skipOnEmpty` is true and no upload is provided

Fixes #2003: Added `filter` property to `ExistValidator` and `UniqueValidator` to support adding additional filtering conditions

Appended newlines to every PHP document that did not have a newline as the last character in the file.

A couple of obvious typos here

Added Model::formName().