Vocabularies used during metadata validation should be logged

[mark-saeon]

We have extended the JSON schema definition with a "vocabulary" keyword, and implemented a corresponding validator which checks that the instance value comes from a vocabulary object defined in CKAN. Since vocabulary objects may mutate independently of the metadata models used for validation (the states of which are logged on the validation activity log record), we should additionally log the id and revision id of any vocabulary object(s) referenced during metadata validation on the activity record.

Note: there is no point really in doing this until Vocabulary and Tag objects are revisioned (which they are not currently in CKAN), since there will be no way to tell from the validation log what the state of a vocabulary was (i.e. what its list of tags was) at the time of a record's validation. A workaround could be to log the entire vocab with a validation activity, but this is far from ideal since we'd be potentially adding tons of duplicate info into the database.

We maybe need to fork CKAN, add revisioning as required, and create an issue and pull request to upstream CKAN.