* Add JSON events API endpoint
* Add regression test for event particulars defaults
* Move JSON events API to a dedicated ActivitiesController
The events endpoint served both the HTML day timeline and the JSON API
feed, but the two paths shared no data or behavior — the HTML side uses
DayTimelinesScoped while the JSON side built its own query. Splitting
into ActivitiesController gives the API its own home at
GET /:account/activities.json without dragging in the timeline
before_actions.
Also preloads comment creator in Event.preloaded to avoid an N+1 when
rendering comment eventables in the JSON feed.
Rob Zolkos