[ticket/12371] Fix post related notifications

- [x] Do not group "quote" notifications …

Grouping these notification makes no sense, as the user will not find the
posts he is quoted in, when they are spread over multiple pages

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- [x] Do not delete post related notifications when a post is deleted

When a post is marked edited and therefor marked as softdeleted/unapproved,
we should not delete the notifications, just like we can't delete the emails.
Links are out there anyway and just keeping the notification is fail save.

If we keep the notification we can just continue to update it when more posts
are added. Also when we delete a post, we don't have to go though the table
and find all related notifications to be able to delete them or even update
them, when they have responders set.

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- [x] Do not add unlimited users as responders

We run into the risc that the data is longer then the character limit
of the table column. However as we trim the users list anyway, we can
also just stop adding them and display "many others" instead of "x others"

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- [x] Do not update the notification entry unneccessarily

When the data did not change, we also don't have to run the query at all.

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https://tracker.phpbb.com/browse/PHPBB3-12371