[ticket/13207] Add default subscription options for newly registered users

PHPBB3-13207
[ticket/13207] Move default user notifications settings to user_add()

PHPBB3-13207
[ticket/13207] Add notification manager mock to user_add method in tests

PHPBB3-13207
Merge remote-tracking branch 'github-marc1706/ticket/13207' into prep-release-3.1.0

* github-marc1706/ticket/13207:
  [ticket/13207] Add notification manager mock to user_add method in tests
  [ticket/13207] Move default user notifications settings to user_add()
  [ticket/13207] Add default subscription options for newly registered users
Merge branch 'prep-release-3.1.0' into develop-ascraeus

* prep-release-3.1.0:
  [ticket/13207] Add notification manager mock to user_add method in tests
  [ticket/13207] Move default user notifications settings to user_add()
  [ticket/13207] Add default subscription options for newly registered users