Add override format query param

lib/clacks_web/plug/format.ex

@@ -4,7 +4,20 @@ defmodule ClacksWeb.Plug.Format do
   def init(opts), do: opts
 
   def call(conn, _opts) do
-    format = Phoenix.Controller.get_format(conn)
+    conn = fetch_query_params(conn)
+
+    format =
+      case conn.query_params do
+        %{"format" => format} when format in ["activity+json", "html"] ->
+          format
+
+        %{"format" => "json"} ->
+          "activity+json"
+
+        _ ->
+          Phoenix.Controller.get_format(conn)
+      end
+
     assign(conn, :format, format)
   end
 end