API allow to create closed milestones (#11745)

* API allow to create closed milestones

* set CloseDate too

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

modules/structs/issue_milestone.go

@@ -28,6 +28,8 @@ type CreateMilestoneOption struct {
 	Description string `json:"description"`
 	// swagger:strfmt date-time
 	Deadline *time.Time `json:"due_on"`
+	// enum: open,closed
+	State string `json:"state"`
 }
 
 // EditMilestoneOption options for editing a milestone