Update Leaflet to 0.6.3
Fix sidebar and top bar, scroll content

Keep the sidebar and top bar fixed at the left/top of
the viewport on pages that scroll (copyright, history).

If the viewport isn't tall enough to show the sidebar,
it scrolls independently.
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.