Send emails via sidekiq (#380)

* add sidekiq web view if the user is an administrator

* add sidekiq setup configuration and support

* update devise to use delivery_later method and update test

* update conversation to use deliver_later instead of deliver

* Update Routes

* Add Procfile for Heroku One-Click Start

* updating docs

* update concurrency and Procfile for supporting Heroku Free Dyno

* update Procfile.dev

config/environments/development.rb

@@ -33,6 +33,8 @@ Rails.application.configure do
   config.active_storage.service = :local
 
   # Don't care if the mailer can't send.
+  config.active_job.queue_adapter = :sidekiq
+  
   config.action_mailer.raise_delivery_errors = false
 
   config.action_mailer.perform_caching = false