Enable asynchronous mail sending in production

[s2t2]

Should be able to GtfsImporMailer.schedule_report(results: results).deliver_later in production and see the mail get delivered asynchronously. Can currently deliver_now, but deliver_later seems to not be working. Need to investigate further and most likely implement a job processing mechanism like ActiveJob / Redis / Sidekiq.

For now the mail is getting delivered synchronously, so this isn't the highest priority.

[s2t2]

Maybe not from rake task? https://edgeguides.rubyonrails.org/active_job_basics.html#action-mailer

[s2t2]

References:

• https://guides.rubyonrails.org/v4.2/active_job_basics.html#job-execution
• https://api.rubyonrails.org/classes/ActiveJob/QueueAdapters/AsyncAdapter.html