Add slogan config (#3752)

This is a PR for #3616

Currently added a new optional config `SLOGAN`  in ini file. When this config is set title page is modified in APP_NAME [ - SLOGAN]

Example in image below

![Selezione_075.png](/attachments/7a72171e-e730-4e57-8c97-ffc94258e00f)

Add the new config value in the admin settings page (readonly)

![Screenshot 2024-05-13 at 18-04-13 My Forgejo.png](/attachments/dad00fc2-29fa-4371-a7b9-5233eadeac13)

## TODO

* [x] Add the possibility to add the `SLOGAN` config from the installation form
* [ ] Update https://forgejo.org/docs/next/admin/config-cheat-sheet

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3752
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: mirko <mirko.perillo@gmail.com>
Co-committed-by: mirko <mirko.perillo@gmail.com>

modules/setting/server_test.go

@@ -0,0 +1,36 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDisplayNameDefault(t *testing.T) {
+	defer test.MockVariableValue(&AppName, "Forgejo")()
+	defer test.MockVariableValue(&AppSlogan, "Beyond coding. We Forge.")()
+	defer test.MockVariableValue(&AppDisplayNameFormat, "{APP_NAME}: {APP_SLOGAN}")()
+	displayName := generateDisplayName()
+	assert.Equal(t, "Forgejo: Beyond coding. We Forge.", displayName)
+}
+
+func TestDisplayNameEmptySlogan(t *testing.T) {
+	defer test.MockVariableValue(&AppName, "Forgejo")()
+	defer test.MockVariableValue(&AppSlogan, "")()
+	defer test.MockVariableValue(&AppDisplayNameFormat, "{APP_NAME}: {APP_SLOGAN}")()
+	displayName := generateDisplayName()
+	assert.Equal(t, "Forgejo", displayName)
+}
+
+func TestDisplayNameCustomFormat(t *testing.T) {
+	defer test.MockVariableValue(&AppName, "Forgejo")()
+	defer test.MockVariableValue(&AppSlogan, "Beyond coding. We Forge.")()
+	defer test.MockVariableValue(&AppDisplayNameFormat, "{APP_NAME} - {APP_SLOGAN}")()
+	displayName := generateDisplayName()
+	assert.Equal(t, "Forgejo - Beyond coding. We Forge.", displayName)
+}