Mail assignee when issue/pull request is assigned (#8546)

* Send email to assigned user

* Only send mail if enabled

* Mail also when assigned through API

* Need to refactor functions from models to issue service

* Refer to issue index rather than ID

* Disable email notifications completly at initalization if global disable

* Check of user enbled mail shall be in mail notification function only

* Initialize notifications from routers init function.

* Use the assigned comment when sending assigned mail

* Refactor so that assignees always added as separate step when new issue/pr.

* Check error from AddAssignees

* Check if user can be assiged to issue or pull request

* Missing return

* Refactor of CanBeAssigned check.

CanBeAssigned shall have same check as UI.

* Clarify function names (toggle rather than update/change), and clean up.

* Fix review comments.

* Flash error if assignees was not added when creating issue/pr

* Generate error if assignee users doesn't exist

models/issue_test.go

@@ -297,7 +297,7 @@ func testInsertIssue(t *testing.T, title, content string) {
 		Title:    title,
 		Content:  content,
 	}
-	err := NewIssue(repo, &issue, nil, nil, nil)
+	err := NewIssue(repo, &issue, nil, nil)
 	assert.NoError(t, err)
 
 	var newIssue Issue