Feature/simple tree

[mudhoney]

• New format=simpletree value on ?action=events: bucket the flat v1 payload into a one-level SOURCE>>Label map keyed by event type. Empty buckets are kept so the client can know the full catalogue regardless of what's in the response.
• The bucketing logic lives in a new Helioviewer\Api\Event\EventTree
• Unknown event paths get a dynamic bucket and are reported to Sentry
• Adds tests
• API docs