Use non-/html search engine URL for DuckDuckGo search plugins

The DuckDuckGo folks asked us to use https://duckduckgo.com instead of the /html variant as the former provides a better search experience. They implemented a redirect to the /html version for users that have JS disabled. Thus, we should be good to go. We should switch the .onion URL as well.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information