* don't update search index if post belongs to deleted topic
* log errors instead of crashing when updating post or revision fails
* update mentions even when the href attribute is missing
* run the background job with low priority
* replace username in all notifications
* update `action_code_who` used by small action posts
* FEATURE: Update avatars in posts and revisions when user gets renamed
* FIX: Replace username in deleted posts when user gets renamed
* FEATURE: Replace username in notifications when user gets renamed
FEATURE: Update mentions and quotes when user gets merged