Show modal when event starts

- Start event template and view
- Show modal when event starts
Delay event starting

- Create delay event view and template
- Add delay event styling
Reschedule Event

- Create reschedule view and template
- Add available times api for event model
Start event
Update announce text to reflect prompt
Issue #41: Prompt to start events

As a user I want to be notified when an event starts with the option to
reschedule it or delay it.

- Show modal when event starts
- Delay event starting
- Reschedule event
- Start event
- Update announce text to reflect prompt