Add a banner for donation during the campaign

From the results of the Tor crowdfunding campaign we learned that the browser banner brought most donations.

This report from the Wikimedia Foundation also brings interesting results:

Longer text brings in more donations than shorter text.
People were convinced by arguments about “no advertising, keeping Wikipedia free, and being a non-profit”.

Feature Branch: web/11297-donation-banner

Original created by @sajolida on 11297 (Redmine)

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