We changed the default retries for Sidekiq jobs from 3 to the upstream
default of 25. When we did this, we manually set all existing workers to
have 3 retries, to avoid changing existing behaviour.

However, mailer jobs aren't run through a worker we define: they go via
ActiveJob. This isn't a huge deal, but typically if a mailer fails
twice, it will fail forever. Also, 25 retries take a few weeks: it's not
very useful to get an email several weeks after it was scheduled to
send.

In Sidekiq 6, ActiveJob will have the ability to use `sidekiq_options`,
but in Sidekiq 5 we'll just get this error:

    You cannot include Sidekiq::Worker in an ActiveJob

Instead, here we manually replace the `#enqueue` and `#enqueue_at`
methods for the Sidekiq adapter to ActiveJob, and special-case those to
give 3 retries for mailer jobs again.

Changelog: fixed