Reproducible website build

Originally created by @intrigeri on #11966 (Redmine)

We include a local copy of the website in the ISO image. It must build reproducibly, somehow. ikiwiki has a deterministic option that should help.

Feature Branch: web/11966-deterministic-website

Edited May 15, 2020 by intrigeri

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