Add welcome page

[Seeking feedback and design attention rather than a merge here -- this is a pre-roll for a pull request against the main osm-website repo.]

This pull request replaces the confirm/welcome email with a dedicated page based on [osmlab/welcome-osm](http://osmlab.github.io/welcome-osm/).

The welcome page is the landing page for newly confirmed accounts as well as a place where third party data consumers like Foursquare, MapBox, or Craigslist can direct users who have spotted a problem with the map and want to get started fixing it.

The intent is to make the introduction of new users to OpenStreetMap gentler than simply linking to the /edit view, while simultaneously curbing the length of the confirmation email -- we can provide a better welcome message through the website than through a static email.

![openstreetmap](https://f.cloud.github.com/assets/98601/806625/eb3265a2-ee3e-11e2-8eae-4fa54989c29a.png)