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
We're trying to move validators out of classes, see ```https://github.com/theforeman/foreman/pull/1943/files``` for examples. Could this be implemented as ```validates :name, :reserved_names => ['Internal', 'Hidden']``` or something similar?