Add summit banners

Closes #576, preview changes:

* http://jekit.codeforamerica.org/codeforamerica/codeforamerica.org/add-summit-banners/summit/
* http://jekit.codeforamerica.org/codeforamerica/codeforamerica.org/add-summit-banners/summit/schedule/
* http://jekit.codeforamerica.org/codeforamerica/codeforamerica.org/add-summit-banners/summit/venue/
* http://jekit.codeforamerica.org/codeforamerica/codeforamerica.org/add-summit-banners/summit/speakers/