Follow-up of art #16124. When all those conditions are met:
- The milestone is created in the Aggregator project
- The mirror milestones are created with the copied informations in the Contributor projects
- Title
- Description
- Status
- Timeframe
- a link toward Aggregator Milestone with '_mirrored_milestone' artifact link type
- The `_mirrored_milestone` artifact link type
- is provided by the multi project backlog plugin
- cannot be set manually at artifact level (same was already done with artifact folder). The only level where it's managed is at Milestone creation
- can be deleted at artifact level
The planning view should be updated for the general usage to order milestone by start date (instead of id)