Implement logout redirection for reverse proxy auth setups (#36085) (#37171)

Backport #36085 by @eliroca

When authentication is handled externally by a reverse proxy SSO
provider, users can be redirected to an external logout URL or relative
path defined on the reverse proxy.

Co-authored-by: Elisei Roca <eroca@suse.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

custom/conf/app.example.ini

@@ -461,6 +461,11 @@ INTERNAL_TOKEN =
 ;; Name of cookie used to store authentication information.
 ;COOKIE_REMEMBER_NAME = gitea_incredible
 ;;
+;; URL or path that Gitea should redirect users to *after* performing its own logout.
+;; Use this, if needed, when authentication is handled by a reverse proxy or SSO.
+;; For example: "/my-sso/logout?return=/my-sso/home"
+;REVERSE_PROXY_LOGOUT_REDIRECT =
+;;
 ;; Reverse proxy authentication header name of user name, email, and full name
 ;REVERSE_PROXY_AUTHENTICATION_USER = X-WEBAUTH-USER
 ;REVERSE_PROXY_AUTHENTICATION_EMAIL = X-WEBAUTH-EMAIL