Replace static event pages with generated versions

[Bertg]

Problem

As per https://github.com/railsgirls/railsgirls/issues/556 generating a new event is painful because the following steps need to happen:

• Create new event static page
• Update index page to show upcoming events
  • These events aren't guaranteed to be ordered correctly
  • They might not even be relevant anymore, and require manual curation to fix these discrepancies
• Update events page with link to newly created event
  • See above

Solution

Move over to using jekyll to build a friendlier and more reliable event information

• [ ] Create contribution guide for adding new events (@csaunders in #562)
• [x] Automatically show events that are upcoming on index
• [ ] Automatically show events on the events page
• [ ] Migrate existing events to new system (@andreionut in #559)
• [ ] Extract basic template to reduce duplication (@mvz in #559)
• [ ] Forward from old URLs to new URLs using meta refresh or Javascript to preserve SEO