Use URI.merge to prevent concatenating two canonical URLs when a custom instance thumbnail was uploaded via AdminFE

lib/pleroma/web/mastodon_api/views/instance_view.ex

@@ -23,7 +23,8 @@ defmodule Pleroma.Web.MastodonAPI.InstanceView do
         streaming_api: Pleroma.Web.Endpoint.websocket_url()
       },
       stats: Pleroma.Stats.get_stats(),
-      thumbnail: Pleroma.Web.base_url() <> Keyword.get(instance, :instance_thumbnail),
+      thumbnail:
+        URI.merge(Pleroma.Web.base_url(), Keyword.get(instance, :instance_thumbnail)) |> to_string,
       languages: ["en"],
       registrations: Keyword.get(instance, :registrations_open),
       approval_required: Keyword.get(instance, :account_approval_required),