provide instance thumbnail at /api/v1/instance (debug)

lib/pleroma/web/endpoint.ex

@@ -12,7 +12,7 @@ defmodule Pleroma.Web.Endpoint do
     at: "/media", from: "uploads", gzip: false
   plug Plug.Static,
     at: "/", from: :pleroma,
-    only: ~w(index.html static finmoji emoji packs sounds sw.js)
+    only: ~w(index.html static finmoji emoji packs sounds instance sw.js)
 
   # Code reloading can be explicitly enabled under the
   # :code_reloader configuration of your endpoint.