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

[John Firebaugh]

Added a fallback for if a default edit location can't be determined by parameters or geolocation:

![image](https://f.cloud.github.com/assets/98601/928461/8a486536-ffb7-11e2-8d26-bcd531a0988f.png)

This is good to merge from my perspective.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/338#issuecomment-22291833
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