Workflow status and Idle event support

[jessepeterson]

Create support for handling various Idle events. Firstly, support workflows receiving an event callback when they configure themselves for receiving an Idle event. But more interestingly:

We now support a new event subscription called IdleNotStartedSince. This intent of this event is to kick-off workflows automatically that have not been started in a configurable amount of time for a given enrollment ID. In other words: regularly run workflows for enrollments that regularly check-in.

To support this we add two new capabilities to the engine. The first is the ability to attach arbitrary data (event context) to an event subscription. For the above new event this means you can include the amount of time, in seconds, since the workflow should have been last run on an enrollment. Secondly we've added the ability of the engine to store, update, and clear the last time an workflow has run for an enrollment. This enables us to be able to check that time stamp and calculate if need to run a workflow again.

Note: this PR has MySQL schema changes. Please see the schema.00001.sql file for modification to apply.

[natewalck]

Non-blocking comment.