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/templates/helper.go

@@ -79,6 +79,12 @@ func NewFuncMap() template.FuncMap {
 		"AppName": func() string {
 			return setting.AppName
 		},
+		"AppSlogan": func() string {
+			return setting.AppSlogan
+		},
+		"AppDisplayName": func() string {
+			return setting.AppDisplayName
+		},
 		"AppSubUrl": func() string {
 			return setting.AppSubURL
 		},