fix redirects on lektor sites (closes #5)

We would much rather have that file in `assets/` but it seems this
wasn't being picked up by lektor, so this will have to do for
now.

This is also how the Jenkins build script was doing it:

https://gitweb.torproject.org/project/jenkins/tools.git/tree/slaves/linux/lektor-website/build#n101

Ideally, we wouldn't have an .htaccess at all since those have
significant performance impact on the webserver, but we just want to
restore service to complete the %"Retire Jenkins" milestone.