Moves the topic timer jobs from being scheduled ahead of time with enqueue_at to a 5 minute scheduled run like bookmark reminders, in a new job called Jobs::EnqueueTopicTimers. Backwards compatibility is maintained by checking if an existing topic timer job is enqueued in sidekiq for the timer, and if it is not running it inside the new job.
The functionality to close/open a topic if it is in the opposite state still remains in the after_save block of TopicTimer, with further commentary, which is used for Open/Close Temporarily.
This also removes the ensure_consistency! functionality of topic timers as it is no longer needed; the new job will always pick up the timers because they are not stored in a fragile state of sidekiq.
Splits the `ToggleTopicClosed` job into two distinct `OpenTopic` and `CloseTopic` jobs to make the code clearer. The old job cannot be deleted yet because of outstanding sidekiq schedules, so a todo has been added to do so later this year.
Also replaced mentions of `topic_status_update` with `topic_timer` in some files, because the `topic_status_update` model is obsolete and replaced by topic timer.
Added some shortcut methods for checking if a topic is open/whether a user can change an open topic.
