feat(admin): add -urlPrefix flag for subdirectory deployment (#8670)

Allow the admin server to run behind a reverse proxy under a
subdirectory by adding a -urlPrefix flag (e.g. -urlPrefix=/seaweedfs).

Closes #8646

weed/admin/handlers/auth_handlers.go

@@ -29,7 +29,7 @@ func (a *AuthHandlers) ShowLogin(w http.ResponseWriter, r *http.Request) {
 	var csrfToken string
 	if err == nil {
 		if authenticated, _ := session.Values["authenticated"].(bool); authenticated {
-			http.Redirect(w, r, "/admin", http.StatusSeeOther)
+			http.Redirect(w, r, dash.P(r.Context(), "/admin"), http.StatusSeeOther)
 			return
 		}
 	} else {