Support references to group milestones and create system notes
When there is a group milestone, it doesn't get linked as a reference. Here, v2.5 is a group milestone:
We should take inspiration from group labels here. In particular, we don't need a reference prefix / suffix for group milestones - we can just look for them if there is no matching project milestone.
When doing this, we should:
- Support references.
- Add a system note when a group milestone is set, using that reference. (IID is tricky here because it could be either the project IID or group IID.)
- Add this to autocomplete.
- Enable slash commands for group milestones.
Edited by Sean McGivern