Use DuckDuckGo for search queries on our website

It’s been discussed a bit on #9900 (closed) but this was years ago and we haven’t improve our internal search in the meantime.

I want to:

• Compare the quality of the results of both on common search queries.
• Check for possible changes on the DuckDuckGo technology/policy (and maybe ask them a favor).
• Lead the discussion on whether we are ok with sharing our users searches with DuckDuckGo.
  • I pointed tails-summit@ here. 2 weeks later, everybody has been in favor of the change.
• Check whether we could still collect data from search queries for UX.
  • This will likely add an HTTP glitch to users, so let's work on this only the next time we'll analyze search queries
• Switch to DuckDuckGo (!498 (merged))
• Get sysadmins to disable the ikiwiki search plugin on the production website
• Ask sysadmins to check whether there's still any reason to keep the cgi support enabled or if our website can finally be fully static
• Triage related tickets
• Translate the placeholder
  • Ask on tails-l10n
  • Ping
  • Fallback on Google Translate
  • German
  • Spanish
  • French
  • Italian
  • Portuguese
  • Russian