send alert when app cannot be reached

[maxheld83]

we need some kind of alert or health check that tells us when users cannot reach the app, or the app becomes unresponsive.
It's one thing if this happens inside shiny, then we get logs.
But if azure is overwhelmed as it was in #315 #309 , then we won't know about this and it'll be hard to debug.

[maxheld83]

@njahn82 just FYI I'm moving this out of the current milestone.
It would be good to have this sort of proactive monitoring in place that tells us when something is wrong, but given other priorities and the current service guarantee of metacheck, this will have to wait.

(I'm also hesitant to invest in this now, b/c it's strictly linked to Azure, and we might still migrate metacheck elsewhere.