Allow each flow to be exposes as a status check

[brendanheywood]

• a drop down box for each flow for what status it should have
• defaults to off, which means it won't have a check at all
• if the flow is a single instance, eg cron, then we want to look at the most recent flow instance
• if the flow is parallel, eg triggered from event, then need some sort of lookback threshold, ie look at the last N instances, or look at the outcome of the instances in the last 24 huors
• expose each flow as its own check instance so each can have its own url and message