✅ proxyUrl for #FedBOX on the server side and for #BOX on the client side.

Now I have to port the server bits which (like I said previously) are quite simple to #ONI, where it's more important to have them for me personally.

Huzzah! Now #ONI also supports proxyUrl, I can finally follow people on instances with authorized fetch, which action was previously failing as the client wouldn't be able to confirm the actor was valid.