better use of event_callback

[Samuel Colvin]

use event_callback more, eg. when opening the enquiry modal.

Use case:

    socket('{{ site.socket_key }}', {
      mode: 'enquiry-modal',
      element: '#socket-enquiry-modal',
      router_mode: 'history',
      event_callback: function() {
        console.log('socket event callback')
        $('[name="client_email"]').each(function(){
          $(this).attr('placeholder', 'Email (Required)')
          $(this).attr('required', 'required')
        })
      }
    })

[Samuel Colvin]

cc @sam-jenkins