Add ability to cancel an event or mark it canceled.

[mfjenn]

In looking at the recent even for Denver RailsBridge, they canceled their event and mentioned in the BT event that there was no way to cancel the event in bridge troll. Looks like we need to add that.

[michaelglass]

requests

• Looking to cancel upcoming event #989
• Cancel upcoming event? #571
• Cancel live event #569
• Remove ElixirBridge Providence 11/11/17 workshop from Site! #570

MVP:

• allow organizers to move event back into "draft" stage

Proposed cancellation flow:

• email everyone that's signed up
• move event into draft stage