feat: Notification on new messages in conversation (#1204)

fixes: #895
fixes: #1118
fixes: #1075

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

db/migrate/20200907094912_rename_user_last_seen.rb

@@ -0,0 +1,5 @@
+class RenameUserLastSeen < ActiveRecord::Migration[6.0]
+  def change
+    rename_column :conversations, :user_last_seen_at, :contact_last_seen_at
+  end
+end