Native group milestones

app/controllers/groups/milestones_controller.rb

app/controllers/projects/milestones_controller.rb

app/finders/issuable_finder.rb

app/finders/milestones_finder.rb

app/helpers/milestones_helper.rb

app/models/concerns/internal_id.rb

app/models/concerns/issuable.rb

app/models/concerns/milestoneish.rb

app/models/dashboard_milestone.rb

app/models/global_milestone.rb

app/models/group.rb

app/models/group_milestone.rb

app/models/milestone.rb

app/services/issuable_base_service.rb

app/services/issues/move_service.rb

app/services/milestones/base_service.rb

app/services/milestones/close_service.rb

app/services/milestones/create_service.rb

app/services/milestones/reopen_service.rb

app/services/milestones/update_service.rb