Automaticallly set created and modified field to vocabularies

[nichtich]

The fields created and modified are only added automatically on mappings but they are also needed for editing vocabularies in BARTOC (PUT and POST to /voc). Moreover it should be forbidden to override these fields except for bulk-import as done via command-line.

[stefandesu]

From #92:

The modified value of a concept scheme would refer to the date either its metadata or one of its concepts was last modified, depending which value is higher.

Originally, I thought we would manually set modified to a value when the source information was modified, but I'd be fine with using the same logic as for mappings. (And as you noted, it's different for bulk import.)

[stefandesu]

Just to clarify and/or make a suggestion for the actual implementation:

• Every time a scheme or its concepts are modified, we will update the modified property on the scheme (will be VERY easy to implement). This would also mean that it's not possible to add one's own modified property because the server will override it every time.
• When a scheme is created, my suggestion would be to use the existing created property or, if it doesn't exist, add the current date as created. Alternatively, we could also override created for every new scheme.*
• When a scheme is supposed to be modified (i.e. PUT/PATCH), we will remove created so that it won't get changed.

*This alternative has an issue: For bulk imports, we don't check for each scheme whether we are creating or modifying it, rather we're just using MongoDB's upsert option that will either create or modify an entity. So if we override created for every scheme, we would update the value for those that are bulk imported and already exist. So I'm voting for the other option where we only change created if it doesn't exist on the scheme.

[nichtich]

There is no reason for changing a an already existing creating other than via bulk import so I agree with the suggestion.