Fixes #6536 Appends the keys of AuthSourceInternal and AuthSourceHidden ...

...by their MD5 hashes to make keywords 'Internal' and 'Hidden' available to Administrators when naming External Auth Sources
@eLobato : If we decide to implement the validation in ```AuthSource`` then the validation is rather complex, with a conditional on the type of the inherited object, hence I do not think it would be possible to use the pre defined validation types to achieve the same. 
But, if we go with adding the validations to all AuthSource models except the```AuthSourceHidden``` and ```AuthSourceInternal```, then of course we can use something like this to achieve what we need.