Enhancement: Threaded email replies for a conversation (#885) (#886)

* Added custom Message-ID and In-Reply-To headers for conversation reply emails
* Added new global config for the default domain (This is used in the above headers)
* Added migration to run the config loader to load the new global config value
* The subject of the conversation reply mailer was made static (This is required for threaded emails)
* Added required tests

config/installation_config.yml

@@ -14,3 +14,5 @@
   value: 'https://www.chatwoot.com/privacy-policy'
 - name: DISPLAY_MANIFEST
   value: true
+- name: FALLBACK_DOMAIN
+  value: chatwoot.com

config/locales/en.yml

@@ -54,3 +54,5 @@ en:
       typical_reply_message_body: "%{account_name} typically replies in a few hours."
       ways_to_reach_you_message_body: "Give the team a way to reach you."
       email_input_box_message_body: "Get notified by email"
+    reply:
+      email_subject: "New messages on this conversation"