Allow to join an open group

Also:

* Refactor interacting with a remote event so that you can interact with
  a remote group as well
* Add a setting for group admins to pick between an open and invite-only
  group
* Fix new groups without posts/todos/resources/events/conversations URL
  set
* Repair local groups that haven't got their
  posts/todos/resources/events/conversations URL set
* Add a scheduled job to refresh remote groups every hour
* Add a user setting to pick when to receive notifications when there's
  new members to approve (will be used when this feature is available)
* Fix pagination for members

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/workers/refresh_groups.ex

@@ -0,0 +1,12 @@
+defmodule Mobilizon.Service.Workers.RefreshGroups do
+  @moduledoc """
+  Worker to build sitemap
+  """
+
+  alias Mobilizon.Federation.ActivityPub.Refresher
+
+  use Oban.Worker, queue: "background"
+
+  @impl Oban.Worker
+  def perform(%Job{}), do: Refresher.refresh_all_external_groups()
+end