Added support for collective tasks

[mjagkow]

• When frontend requests for a new task it can specify the task is "collective".
• Collective tasks are not closed automatically when an interaction has been submitted for them.
• This allows to collect multiple interactions in public groups. This could also be used in personal tasks (e.g. in DM) to allow for multiple responses for one task/message.
• Task can be closed manually by frontend using /api/v1/tasks/close method.