Refactor legacy code for /v2 to support content groups
Use updated content group handling and remove all references to the old `variant` property of contents/answers. The following /v2 endpoints are now fully implemented: * Deleting all contents of a group (/) * Retrieving ids of unanswered content in a group (/unanswered) * Deleting all answers of contents in a group (/answers) * Counting answers of a group (/answercount)