[#139] Send conversation emails (#442)

* [#139] Delayed emails for conversations

* Added the setex and get methods to Redis wrapper
* Set the priorities for the sidekiq queues
* Was not able to use mailhog for testing email in local, switched back to letter opener and added comments on using the SMTP settings
* Added after create hood in messages to queue the sending of mail after 2 minutes using sidekiq worker and also set the redis key for the conversation to avoid the email sending for every message
* Added the sidekiq worker to send the email and delete the conversation redis key
* Added the mailer and mail template
* mailer sends the last 10 messages along with the new messages from the time it was queued

* Send email only in development or if smtp config is set

* Send email only in development or if smtp config is set
* Set the SMTP_PORT in production variable

* Adding redis to circle CI

* Specs for the conversation email changes

* Added specs for conversation email sidekiq worker
* Added specs for conversation mailer
* Added specs in message model for the after create hook for notify email

* Send emails only when there is a reply from agent

* set development to use mailhog

* Adding comments for using letter opener

spec/rails_helper.rb

@@ -5,6 +5,7 @@ require File.expand_path('../config/environment', __dir__)
 abort('The Rails environment is running in production mode!') if Rails.env.production?
 require 'rspec/rails'
 require 'pundit/rspec'
+require 'sidekiq/testing'
 
 # Add additional requires below this line. Rails is not loaded until this point!