Related to: #5305
In order to move all the jobs to Sidekiq, this commit creates a new
Articles/BustMultipleCachesWorker using Sidekiq based on the
Articles/BustMultipleCachesJob (this one is not removed until we know
that any job on this one will be performed).
* fixed `ActiveRecord::NotFound` issue for articles, on article destroy
* extracted ActiveJobs from article callbacks
* added specs for article and reaction deletions
