Add past events page.

[kpearson]

Create a page which only has past events.
Link to it from the bottom of events section on the homepage.

Once we support past events on the site, we will need a filter for the upcoming events page to exclude past events.

The code for the past filter will look something link:

{% for event in site.events %}
  {% if event.date >=  site.time %}
    // event html 
  {% endif %}
{% endfor %}