Use Jekyll Data Files functionality for Sponsors and Speakers

* Adds Gemfile for easier local development

   Use `bundle install` and `jekyll serve --watch`

* Adds a Jekyll site-wide config file

* Replaces JSON-based sponsor list with a Jekyll Data File (YAML-based)

  Updates sponsor template to use the contents of `_data/sponsors.yml`

* Replaces JSON-based speaker list with a Jekyll Data File (YAML-based)

  Updates speaker template to use the contents of `_data/speakers.yml`