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

db/migrate/20200522115645_reload_config.rb

@@ -0,0 +1,5 @@
+class ReloadConfig < ActiveRecord::Migration[6.0]
+  def change
+    ConfigLoader.new.process
+  end
+end