Problem to solve
There is currently no way to list all milestones, only list per group/project.
Current possibilities is to loop over all groups and projects, and then retrieve milestones. Or get all issues and map up milestones (but this wouldn't include milestones without issues).
It would be much more efficient to have an endpoint for milestones.
Add a GET /api/milestones endpoint.
Like the Issues endpoint, the milestones retrieved should default to milestones associated with groups an projects for the authenticated user. But there should be a possibility of scope=all, which retrieves all milestones on the instance.