fix: Make sure custom css gets loaded, fixes #2055

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2338>

front/public/settings.json

@@ -1,4 +1,4 @@
 {
-  "additionalStylesheets": ["/front/custom.css"],
+  "additionalStylesheets": ["/custom.css"],
   "defaultServerUrl": null
 }