Deploy an app to Heroku

streaming logs would probably have to be some kind of background task. Right now that's a little wonky, because you'd have to hand-invent a way to do it with a `herokuLogsStart` and `herokuLogsStop` task pair (a task can't background, so you need tasks that start/stop a background thread). And I think a current problem is that the event sink handed to a task isn't supposed to outlive the task. So @jsuereth I think we have to fix something in sbt server here, probably for `run` logs too - either creating an actual "background task" mechanism, or just allow UIContext event sink to outlive the task so a start/stop task pair can hand-roll a background thread that sends events.