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/graphql/schema/actors/person.ex

@@ -197,5 +197,13 @@ defmodule Mobilizon.GraphQL.Schema.Actors.PersonType do
         {:ok, topic: args.person_id}
       end)
     end
+
+    field :group_membership_changed, :person do
+      arg(:person_id, non_null(:id))
+
+      config(fn args, _ ->
+        {:ok, topic: args.person_id}
+      end)
+    end
   end
 end