feat(backend): add external_urls to GraphQL

Available for :
- saveAdminSettings
- adminSettings
- config

For #1764

lib/graphql/resolvers/config.ex

@@ -88,6 +88,7 @@ defmodule Mobilizon.GraphQL.Resolvers.Config do
       get_in(Application.get_env(:web_push_encryption, :vapid_details), [:public_key])
 
     %{
+      external_urls: Config.external_urls(),
       name: Config.instance_name(),
       registrations_open: Config.instance_registrations_open?(),
       registrations_moderation: Config.instance_registrations_moderation?(),