Create Signals Engine for Sending Messages

[andrewbenington]

What problem would you like to solve? Please describe:

A Signals engine should be created that periodically:

• pulls unscheduled Signals from the signals table and creates an outgoing message for each in the outgoing_signals table
• marks those Signals as scheduled

Either the Signals engine or the existing Messages engine should

• send messages in the outgoing_signals table to the appropriate destination

outgoing_signals are should be kept separate from outgoing_messages to allow Alerts to be prioritized.

Part of #3318