Add relation object for tasks

[philecs]

As a user, I want to create relations between tasks so that I can describe dependencies between them.

The idea is to add a relation object to be able to represent relations between different tasks, such as blocked by or is blocking.

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Acceptance Criteria:

• [x] Relation object idea has been approved or rejected
  • [x] In case of approvement: Relation object has been incroporated into the collaboration model

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Definition of Done