Milestones feature in interventions

[janoliver20]

Interventions also need to adhere milestones so actions can be triggered by using custom milestone dates per participant. For this, the interventions model needs to be extended like the observation model with the milestoneId. And then the algorithm to calculate the trigger time per participant. It just modifies the current event schedule set in the intervention.
For the frontend, the current observation scheduling system with the milestone needs to be used to set the same milestone based schedule.

Acceptance Criteria

• Interventions can be added with a milestone
• It calculates the intervention datetime based on each participants custom milestone
• Tests exist