Migrate embed `Page.Event` to a relational table `PageEventHandlers`

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 #296
Fix tests and old mentions to that code

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