Enhancement: ability to send out an email without summary #1048 (#1049)

Right now as part of conversation continuity, we are using the
ConversationReplyMailer which sends a summary of messages including
the incoming messages when an agent replies. Ideally, we want
to send only the reply of that agent and not a summary when
Conversation continuity is enabled. Added the functionality
to send the reply email without summary. Added required unit
tests to cover the changes.

ref: #1048

spec/factories/messages.rb

@@ -2,7 +2,7 @@
 
 FactoryBot.define do
   factory :message do
-    content { 'Message' }
+    content { 'Incoming Message' }
     status { 'sent' }
     message_type { 'incoming' }
     content_type { 'text' }