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/user.go

@@ -1320,16 +1320,20 @@ func GetUsersByIDs(ids []int64) ([]*User, error) {
 }
 
 // GetUserIDsByNames returns a slice of ids corresponds to names.
-func GetUserIDsByNames(names []string) []int64 {
+func GetUserIDsByNames(names []string, ignoreNonExistent bool) ([]int64, error) {
 	ids := make([]int64, 0, len(names))
 	for _, name := range names {
 		u, err := GetUserByName(name)
 		if err != nil {
-			continue
+			if ignoreNonExistent {
+				continue
+			} else {
+				return nil, err
+			}
 		}
 		ids = append(ids, u.ID)
 	}
-	return ids
+	return ids, nil
 }
 
 // UserCommit represents a commit with validation of user.