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>

test/support/factory.ex

@@ -30,6 +30,7 @@ defmodule Mobilizon.Factory do
       notification_each_week: false,
       notification_before_event: false,
       notification_pending_participation: :one_day,
+      notification_pending_membership: :one_day,
       user_id: nil
     }
   end