Smarter announcement preview on home page

[ct-martin]

Have logic for all posts within one or two weeks, or the latest post if none within that timeframe.

@jrtechs @Tjzabel would you rather a one or two week preview period?

[ct-martin]

Also remember that Jekyll is a static generator, not a dynamic one, so this will only get updated when the site gets rebuilt by a new commit.

[jrtechs]

I think a two week window would be fine. Since it will be rebuilt whenever a new announcement is posted, I think that it being static will work out just fine for us.