Migrate embed Page.Event to a relational table PageEventHandlers

[leandrocp]

We need to migrate the existing embed Page.Event into its own table beacon_page_event_handlers and schema Beacon.Content.PageEventHandler, similar to how #296 was implemented.

• Remove

  beacon/lib/beacon/content/page.ex

  Lines 47 to 50 in 40fa03e

  	embeds_many :events, Event do
  	field :name, :string
  	field :code, :string
  	end

• Create table
• Create schema
• Create has_many :event_handlers association in Content.Page
• Preload :event_handlers association like was done on PR Add page variants #296
• Fix tests and old mentions to that code