EZP-21515: removed UserMetadata::CREATOR

The original creator of a content object is something that we might not
have in legacy since archived versions might be removed. Therefore, we
remove that notion for now. It might be reintroduced with an enhancement
request.
Fixed EZP-21515: Missing Legacy Converter for UserMetaData Criterion
EZP-21515: extra documentation about VersionInfo->creatorId being the 'modifier'
Merge pull request #501 from patrickallaert/EZP-21515

Fixed EZP-21515: Missing Legacy Converter for UserMetaData Criterion