Schedule support

The [schedule branch](https://github.com/CTPUG/wafer/tree/schedule) is now at a point were it'll benefit from some other people looking at it.

I'm not certain is the current start_time / previous_slot & end_time approach to specifying slots is the best approach - it might be better to go with start_time / previous_slot and slot length.

The extra validation information stuffed into the admin form probably needs some design TLC.

The actual schedule should also check for valiation issues and not show invalid or incomplete schedules.