fix broken links

This pull request includes three (seemingly unrelated) things, but each one made the next one easier.

1. Change the URL structure to use directories instead of missing file extensions
    * In other words, now `/open-source-program/pages/faq/` instead of `/open-source-program/pages/faq/`.
    * This now means that links work when running `jekyll server` locally.
    * Luckily links to us won't be broken, because GitHub Pages handles that redirect.
    * See demo at http://afeld.github.io/open-source-program/ (which is showing this branch)
1. Set up a Rake task for checking for broken links
    * We could set up Travis to run this, but it's a bit annoying if someone submits a PR to make a spelling change and their build fails because an external link timed out. Open to it if people want, though.
1. Fix broken links (internal + external)