Add course material in a maintainable way, include more file extensions, and set up an example participant blog

This pull request adds the course material in a way that will allow it to automatically aggregate in the appropriate places based on three categories: `policies`, `daily_agendas`, and `weekly_overviews`. These materials are now linked individually on the course materials page, as well as having their own category aggregates, e.g., all policies can be seen in one place. Also, policies now have prettified permalinks.

It also modifies the `_config.yml` file to allow Jekyll to interpret files with multiple markdown extensions. Previously, the site would only appropriately render files with a `.markdown` extension.

It also sets up an example blog structure under @lsblakk's participant folder that allows for posts and drafts, as well as each participant to have their blogs both aggregate into the main blog feed, as well as have an individual page that shows just their posts under `http://ascendproject.org/participants/portland/participant-name/`.