Add tickets link to schedule when eventyay-tickets component is active

[lcduong]

This PR closes/references issue #124 . It does so by:

• Add new input form in general setting of event.
• If Ticket link is configured, showing in event pages

How has this been tested?

Checklist

• [x] I have added tests to cover my changes.

[lcduong]

New input form under general setting of event.

If this form is filled, showing like below screenshot in event page.

When click 'Tickets', user will be redirect to ticket shop.