Add @ prefix for sender username in push notifications (#11934)

* Add @ prefix for sender username in push notifications

* de-duplicating code

model/user_test.go

@@ -252,6 +252,42 @@ func TestUserGetDisplayName(t *testing.T) {
 	}
 }
 
+func TestUserGetDisplayNameWithPrefix(t *testing.T) {
+	user := User{Username: "username"}
+
+	if displayName := user.GetDisplayNameWithPrefix(SHOW_FULLNAME, "@"); displayName != "@username" {
+		t.Fatal("Display name should be username")
+	}
+
+	if displayName := user.GetDisplayNameWithPrefix(SHOW_NICKNAME_FULLNAME, "@"); displayName != "@username" {
+		t.Fatal("Display name should be username")
+	}
+
+	if displayName := user.GetDisplayNameWithPrefix(SHOW_USERNAME, "@"); displayName != "@username" {
+		t.Fatal("Display name should be username")
+	}
+
+	user.FirstName = "first"
+	user.LastName = "last"
+
+	if displayName := user.GetDisplayNameWithPrefix(SHOW_FULLNAME, "@"); displayName != "first last" {
+		t.Fatal("Display name should be full name")
+	}
+
+	if displayName := user.GetDisplayNameWithPrefix(SHOW_NICKNAME_FULLNAME, "@"); displayName != "first last" {
+		t.Fatal("Display name should be full name since there is no nickname")
+	}
+
+	if displayName := user.GetDisplayNameWithPrefix(SHOW_USERNAME, "@"); displayName != "@username" {
+		t.Fatal("Display name should be username")
+	}
+
+	user.Nickname = "nickname"
+	if displayName := user.GetDisplayNameWithPrefix(SHOW_NICKNAME_FULLNAME, "@"); displayName != "nickname" {
+		t.Fatal("Display name should be nickname")
+	}
+}
+
 var usernames = []struct {
 	value    string
 	expected bool