Remove https://tails.boum.org/recentchanges

Initial problem statement

Right now, it's only linked from:

• https://tails.boum.org/contribute/how/code/

  This section could be improved and point to GitLab instead. Honestly, who's going to follow our work using this RSS feed?

• https://tails.boum.org/contribute/release_process/

  To check when the website is done publishing a new release. We can probably find something else.

But it's indexed by DuckDuckGo and our internal search engine and pollute search results, see #17652 (closed).

Tasks

• Remove links to recentchanges from /contribute
• Disable the recentchanges plugin on our production website
• Remove wiki/src/recentchanges.mdwn
• Remove obsolete references to it: git grep recentchanges
• Ensure recentchanges/* don't appear in search results anymore