[#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

.circleci/config.yml

@@ -13,6 +13,7 @@ defaults: &defaults
     # CircleCI maintains a library of pre-built images
     # documented at https://circleci.com/docs/2.0/circleci-images/
     - image: circleci/postgres:9.4
+    - image: circleci/redis:5.0.7-alpine
   environment:
       - CC_TEST_REPORTER_ID: b1b5c4447bf93f6f0b06a64756e35afd0810ea83649f03971cbf303b4449456f