tb-manual: create redirects for old resources

The Tor Browser User Manual has been ported to lektor, and it's been updated and localized.

But the final mapping of the contents varies a bit: What before was https://tb-manual.torproject.org/en-US/secure-connections.html now is https://tb-manual.torproject.org/en-US/secure-connections , and the previous link is giving a 404.

I think we should have

• a redirect that goes: if 404, then strip '.html' and try again.
• a way to monitor the 404 errors for a while to be able to map more redirections. This resources have been widely linked and we should make sure we can keep those links.