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>

config/config.exs

@@ -263,7 +263,8 @@ config :mobilizon, Oban,
   log: false,
   queues: [default: 10, search: 5, mailers: 10, background: 5],
   crontab: [
-    {"@hourly", Mobilizon.Service.Workers.BuildSiteMap, queue: :background}
+    {"@hourly", Mobilizon.Service.Workers.BuildSiteMap, queue: :background},
+    {"17 * * * *", Mobilizon.Service.Workers.RefreshGroups, queue: :background}
   ]
 
 config :mobilizon, :rich_media,