A whole bunch of changes this time, the most important of which is to use the
'device_model' AND 'device_serial' as unique key to locating the NotifyControl.
This required adding a device_model column to notify_controls table.

Defined association between Alert and NotifyControl so that Alert belongs_to
NotifyControl and NotifyControl has_many Alerts. This required adding the
notify_control_id(integer) column to alerts table.

These two changes resulted in changes to a bunch of views and controllers.