Feature: Inbox greeting message (#927)

Co-authored-by: Pranav Raj S <pranav@thoughtwoot.com>

app/models/inbox.rb

@@ -7,6 +7,8 @@
 #  id                     :integer          not null, primary key
 #  channel_type           :string
 #  enable_auto_assignment :boolean          default(TRUE)
+#  greeting_enabled       :boolean          default(FALSE)
+#  greeting_message       :string
 #  name                   :string           not null
 #  created_at             :datetime         not null
 #  updated_at             :datetime         not null