Timebox events

[graygilmore]

As an event organizer,
I would like presentations to total no more than 50min combined,
So that we can make sure we don't run long.

Acceptance Criteria

• Given I have an event with presentations totaling 45min
  • and I try to add an event 10min long
    • I should receive an error telling me that there is not enough time for a 10min presentation
  • and I try to add an event 5min long
    • I should not receive an error and the presentation should be added to the event