[openstreetmap-website] Add Donation Drive Banner (#284)

[Grant]

Add donation drive banner.
Thank you to ris for creating banner.
You can merge this Pull Request by running:

  git pull https://github.com/Firefishy/openstreetmap-website master

Or you can view, comment on it, or merge it online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/284

-- Commit Summary --

  * Add Server Donation Banner

-- File Changes --

    A app/assets/images/donate-ad.png (0)
    M app/views/layouts/site.html.erb (5)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/284.patch
https://github.com/openstreetmap/openstreetmap-website/pull/284.diff

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