Switch to the DuckDuckGo .onion by default

Originally created by @Kurtis on #12121 (Redmine)

A little bit of discussion about this can be found here: https://labs.riseup.net/code/issues/6059

The last Tails release seemed to have removed the DuckDuckGO onion service as secondary search option. Was there a security or other reason why it was removed and why the HTTPS version was used?

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