Adding more structured fields for event descriptions

[aeschright]

I have some ideas about some optional fields that we could add to the event listings that would be helpful for organizers and attendees. Two in particular:

Code of conduct

We could have a spot for a link to the group's code of conduct. I'm not sure if we'd want to call out the lack of one -- it might be better to have some information about implementing a CoC on the new event page.

Registration

With Meetup, workshops, and conference listings, there's often a registration link that people need to go to before they can attend. We could make this a little easier by creating a consistent spot for it to appear in the listing.