feat: limit attendees

[enricoprazeres]

• Created constants.
• Changed the dashboard to a new backoffice page.
• It just shows a flash and blocks the registration.
• Open registrations constant.
• Shows only the login button when registrations are closed (max attendees reached or closed manually).
• Stops the payment when the team size + attendees with paid teams reaches the max attendees for the event.

[nunom27]