send bulk e-certificate to all participant and speaker

[jacky-htg]

• page for admin only
• from the page of detail event, if event is done, show the button of "Send E-Certificate".
• when the button is click, show the modal confirmation "Have you marked the attendance of participants? because only participants who attend will be sent a certificate. And are you sure you want to continue sending the e-certificate?"
• if admin confirm this process, send bulk email to all attendees and speaker.
• update table of events, set has_send_certificate = true.