Add simple way to decode fully qualified mediaproxy URLs

lib/pleroma/web/media_proxy.ex

@@ -121,6 +121,11 @@ def decode_url(sig, url) do
     end
   end
 
+  def decode_url(encoded) do
+    [_, "proxy", sig, base64 | _] = URI.parse(encoded).path |> String.split("/")
+    decode_url(sig, base64)
+  end
+
   defp signed_url(url) do
     :crypto.hmac(:sha, Config.get([Web.Endpoint, :secret_key_base]), url)
   end