feat(backend): add external_urls to GraphQL

Available for :
- saveAdminSettings
- adminSettings
- config

For #1764

lib/graphql/resolvers/admin.ex

@@ -280,6 +280,11 @@ defmodule Mobilizon.GraphQL.Resolvers.Admin do
          :ok <- eventually_update_instance_actor(res) do
       Config.clear_config_cache()
 
+      # add external_urls
+      urls = Map.get(res, :external_urls, [])
+      external_urls = Enum.map(urls, &Config.transform_external_url/1)
+      res = Map.put(res, :external_urls, external_urls)
+
       {:ok, res}
     end
   end