provide instance thumbnail at /api/v1/instance

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -103,6 +103,7 @@ def masto_instance(conn, _params) do
         streaming_api: String.replace(Web.base_url, ["http","https"], "wss")
       },
       stats: Stats.get_stats,
+      thumbnail: Web.base_url <> "/static/instance-thumbnail.jpeg",
       max_toot_chars: Keyword.get(@instance, :limit)
     }