Meta thread: https://meta.discourse.org/t/cant-dismiss-unread-if-last-post-is-an-assign-or-whisper/131823/7
* when sending a whisper, the highest_staff_post_number is set
in the next_post_number method for a Topic, but the
highest_post_number is left alone. this leaves a situation
where highest_staff_post_number is > highest_post_number
* when TopicsBulkAction#dismiss_posts was run, it was only setting the topic_user
highest_seen_post_number using the highest_post_number from the topic, so if
the user was staff and the last post in a topic was a whisper
their highest seen number was not set, and the topic stayed unread
Found through testing that the bug wasn't to do with Assign/Unassign as they do not affect the post numbers, only whispering does.
f753643cb1