'Organizer' field of Event is optional.
'Organizer' field of Event is optional.

The `Event` model allows the `organizer` field to be NULL, but the form wasn't allowing it to be empty.  This PR fixes that by allowing `organizer` to be left empty in the event creation form.

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