Add future events

[marksteward]

Future events are as simple as a new post, but we need to only show the next one.

Also, from @Jonty in #9:

I'll submit a PR that adds an automatic ical feed (though this isn't a blocker)

[Svenito]

the front page should already show only the latest post (by post date).

[tonynaggs]

I would like to have a list of upcoming event dates.
(Possibly with a link to our Google Calendar, so that people can easily subscribe to that.)

People like to see this, also it would be good to have a canonical list that can be used to update the new Defcon Groups site calendar:
https://defcongroups.org/calendar.html