Add workshops

[Abrikot]

As of now, we can create events only for competitors as the "Events" page is only displayed to competitors. That would be great to have a way to list all available workshops so that non-competitors could register themselves.

I think the best way to display that information in the front-end would be a whole new tab named "Workshops". This could either be linked to a new back-end page, similar to convention_setup/categories or this could reuse the current convention_setup/categories page with a checkbox for all events that would tell whether the event is a competition or a workshop.