[ticket/12370] Do not delete topic notifications when the topic is visible

We should only delete the topic notifications, when the topic is not approved
anymore. This happens, when the post was the last approved but is now
unapproved because the user is on queue, or when it has been softdeleted while
editing.

PHPBB3-12370
[ticket/12370] Fix functional notification test and remove unneeded requests

PHPBB3-12370
[ticket/12370] Add unit tests for topic notifications

PHPBB3-12370
Merge remote-tracking branch 'github-nickvergessen/ticket/12370' into develop-ascraeus

* github-nickvergessen/ticket/12370:
  [ticket/12370] Add unit tests for topic notifications
  [ticket/12370] Fix functional notification test and remove unneeded requests
  [ticket/12370] Do not delete topic notifications when the topic is visible