Website build takes 10-20 minutes longer since upgrade to 3.20160905~bpo8+1

On November 19 we upgraded ikiwiki on our isobuilders from 3.20141016.3 to 3.20160905~bpo8+1 (a459578, for #11966 (closed), but it was going to happen at some point anyway). Since then, ISO build times went crazy because the web site takes much more time to build.

Feature Branch: web/12053-faster-build

Original created by @intrigeri on 12053 (Redmine)

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