Move reverproxyauth before session so the header will not be ignored even if user has login (#27821) (#30947)

Backport #27821 by @lunny

When a user logout and then login another user, the reverseproxy auth
should be checked before session otherwise the old user is still login.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/web.go

@@ -100,14 +100,14 @@ func optionsCorsHandler() func(next http.Handler) http.Handler {
 // The Session plugin is expected to be executed second, in order to skip authentication
 // for users that have already signed in.
 func buildAuthGroup() *auth_service.Group {
-	group := auth_service.NewGroup(
-		&auth_service.OAuth2{}, // FIXME: this should be removed and only applied in download and oauth related routers
-		&auth_service.Basic{},  // FIXME: this should be removed and only applied in download and git/lfs routers
-		&auth_service.Session{},
-	)
+	group := auth_service.NewGroup()
+	group.Add(&auth_service.OAuth2{}) // FIXME: this should be removed and only applied in download and oauth related routers
+	group.Add(&auth_service.Basic{})  // FIXME: this should be removed and only applied in download and git/lfs routers
+
 	if setting.Service.EnableReverseProxyAuth {
-		group.Add(&auth_service.ReverseProxy{})
+		group.Add(&auth_service.ReverseProxy{}) // reverseproxy should before Session, otherwise the header will be ignored if user has login
 	}
+	group.Add(&auth_service.Session{})
 
 	if setting.IsWindows && auth_model.IsSSPIEnabled() {
 		group.Add(&auth_service.SSPI{}) // it MUST be the last, see the comment of SSPI