[openstreetmap-website] Add welcome page (#338)

[John Firebaugh]

After discussions with @lxbarth, we decided that the welcome page would be better if it didn't try to serve dual purposes as a landing page both for first time visitors without an account _and_ users who had just signed up. Accordingly, in the most recent iteration, I split up the content.

A very short bit of introductory text has been moved to the sign up page:

![image](https://f.cloud.github.com/assets/98601/857281/d704d720-f54d-11e2-9b9f-c2a7ffd3b6a7.png)

The welcome page is now tailored for first-time, logged-in users:

![image](https://f.cloud.github.com/assets/98601/857287/0b08ada8-f54e-11e2-9416-5e57118cb02f.png)

The new design removes the contextualization of OpenStreetMap within a data consumer ecosystem and does not mention any organizations by name. We'll leave that to a future "about" page or to third-parties themselves.

One remaining problem is that there's no clear location for the "Start Mapping" button to start at if the browser doesn't support geolocation and no lat/lon query parameters have been provided. This will be less of an issue when iD is the default editor, because it can be opened at a whole-world view and the user can zoom in from there, but P2 doesn't support that.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/338#issuecomment-21570065
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