Refactor user & avatar (#33433)

1. better GetPossibleUserByID logic
2. fix some function name & comment typos
3. do not re-generate avatar if one exists

models/user/user_system.go

@@ -41,6 +41,10 @@ const (
 	ActionsUserEmail = "teabot@gitea.io"
 )
 
+func IsGiteaActionsUserName(name string) bool {
+	return strings.EqualFold(name, ActionsUserName)
+}
+
 // NewActionsUser creates and returns a fake user for running the actions.
 func NewActionsUser() *User {
 	return &User{
@@ -58,6 +62,16 @@ func NewActionsUser() *User {
 	}
 }
 
-func (u *User) IsActions() bool {
+func (u *User) IsGiteaActions() bool {
 	return u != nil && u.ID == ActionsUserID
 }
+
+func GetSystemUserByName(name string) *User {
+	if IsGhostUserName(name) {
+		return NewGhostUser()
+	}
+	if IsGiteaActionsUserName(name) {
+		return NewActionsUser()
+	}
+	return nil
+}