[#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/views/conversation_mailer/new_message.html.erb

@@ -0,0 +1,16 @@
+<p>Hi <%= @contact.name %>,</p>
+
+<p>You have new messages on your conversation.</p>
+
+<table>
+    <% @messages.each do |message| %>
+        <tr>
+            <td>
+                <b><%= message.incoming? ? 'You' : message.user.name %></b>
+            </td>
+            <td>: <%= message.content %></td>
+        </tr>
+    <% end %>
+</table>
+
+<p>Click <%= link_to 'here', app_conversation_url(id: @conversation.display_id) %> to get back to the conversation. </p>