Force all 11.5 users to update through 11.5.8 before 12.0

This will require keeping a copy of 11.5.8 mar files on the mirrors for a while. By the way I think we still have some pre 8.0 mar files there that we could remove.

The mechanism to do that is copying the 11.5.8 update xml files from https://aus1.torproject.org/torbrowser/update_3/release/ to an other directory like https://aus1.torproject.org/torbrowser/update_pre12.0/release/, and before publishing the 12.0 update, edit projects/release/update_responses_config.yml to add some redirect rules in htaccess_rewrite_rules, to redirect users of 11.* to update_pre12.0.