Fix: error with wrong reverse URL matching
Fix: error with wrong reverse URL matching

Event validate was taking `event.slug`, but it's empty for
not-yet-finalized workshops.  Thus I changed the template to use
`event.id` (both could be used because of URL regex, but only `pk` is
available when event is not published).

vahtras/amy,shapiromatron/amy,swcarpentry/amy,wking/swc-amy,shapiromatron/amy,wking/swc-amy,shapiromatron/amy,pbanaszkiewicz/amy,wking/swc-amy,pbanaszkiewicz/amy,vahtras/amy,swcarpentry/amy,wking/swc-amy,vahtras/amy,pbanaszkiewicz/amy,swcarpentry/amy