Better homepage

This is an updated version of the homepage including:

- a more informative blurb at the top of the page that lists counts of members, crops, etc
- a display of some of our users (interesting ones picked based on various attributes)
- a list of recent plantings and posts

Basically the point is to make the front page more dynamic and interesting for non-signed-in visitors and give them useful things to click on to get deeper into the site.

In doing this, we've incidentally fixed up timestamp display on posts/comments as well.