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:

1. Support references.
2. 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.)
3. Add this to autocomplete.
4. Enable slash commands for group milestones.