Add JSON activities API endpoint (#2783)

* 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.

config/routes.rb

@@ -133,6 +133,7 @@ Rails.application.routes.draw do
     end
   end
 
+  resources :activities, only: :index
   resources :events, only: :index
   namespace :events do
     resources :days