Add a pre-populated "Submit an event" link on the "Events" page

This pull requests adds a "Add an event" button which redirects to a pre-populated GitHub issue. This allows contributors to submit event more easily, while still being efficient for maintainers.

![capture du 2014-08-03 14 44 37](https://cloud.githubusercontent.com/assets/3578089/3793070/1e44e0f8-1b8b-11e4-8a97-0c75e3abed50.png)