Serve main pages without .html extension

This started off as a quick fix for https://github.com/belfastruby/belfastruby.com/issues/18, by moving the resources and past events pages to be `index.html` files in subdirectories, but also ended up including a few other tweaks too:

- Using logic in Liquid to compute which nav menu links get `class="active"` instead of applying the class via JS.
- Only showing events with `past: true` set on the "past events" page.