Forum bug: Moving a thread between categories

[Lezek123]

The current move_thread_to_category implementation is not updating thread.category_id, which is causing multiple issues when moving a thread between categories.

For example, let's assume we have two categories: 1 and 2, and a single thread in category 1

1. We move the thread from category 1 to category 2. This works as expected so far, the threadById map is updated and the num_direct_threads counter for each category is correct (0 and 1 respectively)
2. We move the thread from category 2 back to category 1. Because thread.category_id is still 1 after the first step, the thread is not actually removed from category 2 and ends up duplicated (appears in both categories). Furthermore, the num_direct_threads of category 1 is first decremented and then incremented, causing it to remain at 0.
3. We move the thread from category 1 to category 2 again. The thread is now removed from category 1 and the num_direct_threads in category 1 is decremented again, causing the counter to overflow to 4294967295. The num_direct_threads is incremented causing it to become 2, which is also invalid.

┆Issue is synchronized with this Asana task by Unito