Eventyay-Common: Add option to create video access to the event

[mariobehling]

During event create organisers currently have the option to create the event for the ticket and talk module. Add the option to add a video module to the event. Each event should have a "video world".

How to deal with the question of "video admin"? Event creators should be admins. How can they assign more admins?

Related to: fossasia/eventyay-video#240