GenericContent reference fields should be limited to type

[enikonemeth]

Now, GenericContent has some reference fields which have not limited to types. Eg.: CreatedBy, VersionCreatedBy, ModifiedBy. We should check these reference fields and guess the types of them: User, SnaasUser? or any descendant of User type?

The exact task should be fill the Allowedtypes in the generic content's CTD:

<Configuration>
   <AllowedTypes>
      <Type>User</Type>
   </AllowedTypes>
</Configuration>