automatically mark events as passed

[dannyhope]

…instead of relying on someone to remember to add upcoming:no to the front matter.

[alexandtheweb]

@dannyhope Jekyll's static. Which means we'll always have to do a build on the site when an event expires. See jekyll/jekyll#2574

But we CAN avoid expiring posts using front matter flags.

[dannyhope]

@alexandtheweb it makes sense to have to regenerate the site but atleast that’s a pretty simple task