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/federation/activity_pub/audience.ex

@@ -4,7 +4,7 @@ defmodule Mobilizon.Federation.ActivityPub.Audience do
   """
 
   alias Mobilizon.Actors
-  alias Mobilizon.Actors.Actor
+  alias Mobilizon.Actors.{Actor, Member}
   alias Mobilizon.Discussions.{Comment, Discussion}
   alias Mobilizon.Events.{Event, Participant}
   alias Mobilizon.Share
@@ -150,6 +150,12 @@ defmodule Mobilizon.Federation.ActivityPub.Audience do
     %{"to" => [participant.actor.url], "cc" => actor_participants_urls}
   end
 
+  def calculate_to_and_cc_from_mentions(%Member{} = member) do
+    member = Repo.preload(member, [:parent])
+
+    %{"to" => [member.parent.members_url], "cc" => []}
+  end
+
   def calculate_to_and_cc_from_mentions(%Actor{} = actor) do
     %{
       "to" => [@ap_public],