Oops, only redirect to maintenance.html internally

Oh I see, if I start with a slash, then it's interpreted as a reference
to a file; whereas if I don't, it's interpreted as a URL redirect. Ok!

deploy/files/sites-available/impress.conf

@@ -34,7 +34,7 @@ server {
   }
 
   # On status 503, return the maintenance page.
-  error_page 503 maintenance.html;
+  error_page 503 /maintenance.html;
 
   # Try serving static files first. If not found, fall back to the app.
   try_files $uri/index.html $uri @app;