Move more issue assignee code from models to issue service (#8690)

* Move more issue assignee code from models to issue service

* fix test

services/issue/assignee.go

@@ -0,0 +1,53 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package issue
+
+import (
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/notification"
+)
+
+// DeleteNotPassedAssignee deletes all assignees who aren't passed via the "assignees" array
+func DeleteNotPassedAssignee(issue *models.Issue, doer *models.User, assignees []*models.User) (err error) {
+	var found bool
+
+	for _, assignee := range issue.Assignees {
+
+		found = false
+		for _, alreadyAssignee := range assignees {
+			if assignee.ID == alreadyAssignee.ID {
+				found = true
+				break
+			}
+		}
+
+		if !found {
+			// This function also does comments and hooks, which is why we call it seperatly instead of directly removing the assignees here
+			if _, _, err := ToggleAssignee(issue, doer, assignee.ID); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}
+
+// ToggleAssignee changes a user between assigned and not assigned for this issue, and make issue comment for it.
+func ToggleAssignee(issue *models.Issue, doer *models.User, assigneeID int64) (removed bool, comment *models.Comment, err error) {
+	removed, comment, err = issue.ToggleAssignee(doer, assigneeID)
+	if err != nil {
+		return
+	}
+
+	assignee, err1 := models.GetUserByID(assigneeID)
+	if err1 != nil {
+		err = err1
+		return
+	}
+
+	notification.NotifyIssueChangeAssignee(doer, issue, assignee, removed, comment)
+
+	return
+}