[#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

app/workers/conversation_email_worker.rb

@@ -0,0 +1,15 @@
+class ConversationEmailWorker
+  include Sidekiq::Worker
+  sidekiq_options queue: :mailers
+
+  def perform(conversation_id, queued_time)
+    @conversation = Conversation.find(conversation_id)
+
+    # send the email
+    ConversationMailer.new_message(@conversation, queued_time).deliver_later
+
+    # delete the redis set from the first new message on the conversation
+    conversation_mail_key = Redis::Alfred::CONVERSATION_MAILER_KEY % @conversation.id
+    Redis::Alfred.delete(conversation_mail_key)
+  end
+end