Add welcome page

This pull request introduces a dedicated welcome 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.

As part of this change, the account confirmation email has been reduced to a single confirmation link, which will then redirect to the welcome page.

[View the page on a test deployment](http://welcome.apis.dev.openstreetmap.org/welcome)