Add welcome page based on http://osmlab.github.io/welcome-osm/
Reduce welcome email to minimal text

Instead, redirect newly-confirmed users to the welcome
page. If confirmation is not required, redirect immediately
and don't send an email.
Adjust whitespace and color
Copy updates

* Less !!!
* Discourage copying from other sources
* Don't mention creating a profile page
* Don't explicitly instruct users to ask questions
* Cut editor list
* Refine note instructions
Add a "Start Mapping" button

Position is determined in one of two ways: lat/lon query
parameters (with helpful text from a nominatim reverse
geocode), or HTML5 geolocation.
More wordsmithing
Make all names translatable
Fix help icon in sprite
Update note instructions for new map UI
Update names, pick via random sample
Wordsmithing
Combine and refine third and sixth section
Move pre-sign up content to sign up page