DEV: Add `post_alerter_after_save_post` event (PR #14388)

Also promote the create_notification_alert and push_notification methods from instance methods to class methods so that plugins can call them. This is temporary until we add a more comprehensive API for extending PostAlerter.


You might want to add includes(:do_not_disturb_timings) to the user’s query here

Done @markvanlan. There are probably tens if not hundreds of optimizations like this in PostAlterter, but we don’t notice them because it’s mostly called in Sidekiq.