Use ikiwiki 3.20161219+, again

I did this on #12051 (closed) already, but then my work was lost by some conflict merge resolution (presumably when we moved the build deps installation from one script to another). So right now our ISO build Vagrant systems use ikiwiki from Jessie, instead of from jessie-backports as intended.

Feature Branch: bugfix/13531-ikiwiki-from-jessie-backports

Parent Task: #5630 (closed)

Related issues

Related to #11966 (closed)
Related to #12051 (closed)

Original created by @intrigeri on 13531 (Redmine)

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