[openstreetmap-website] Add 'x' to close welcome box; remember preference (#582)

[Tom Hughes]

Tweaked a bit and merged as 640ff57465b5288a183e1343285c1550b1a0d1a8.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/582#issuecomment-29583823
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