check last_write_at field when applying rate limits to channels

db/migrate/20150311201046_add_last_write_at_to_channels.rb

@@ -0,0 +1,6 @@
+class AddLastWriteAtToChannels < ActiveRecord::Migration
+  def change
+    add_column :channels, :last_write_at, :datetime
+  end
+end
+