Add welcome page

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. The welcome page fills a gap in the existing introductory materials: existing 'Learn OSM' projects are either tilted towards case (HOT's learnosm.org is for field mappers, most YouTube videos are about JOSM), or more detailed and technical than is desirable for a first experience landing page.

![](https://f.cloud.github.com/assets/98601/806625/eb3265a2-ee3e-11e2-8eae-4fa54989c29a.png)
![](https://f.cloud.github.com/assets/98601/806471/71709b24-ee3b-11e2-8d8f-64618b6f51a4.png)