The `services.mastodon` module currently hardcodes sidekiq's concurrency
to 25, but doesn't set a DB pool size, which defaults to 5 or the number
of configured web threads.
(This behaviour is very strange, and arguably a mastodon bug.)
This also makes sidekiq's concurrency configurable, because 25 is a tad
high for the hardware I'm running it on.
(cherry picked from commit e8fd7792d1eeb4ea4943cc34525da1159ab50bc9)
4f8927c417