Hotfix: Import sprintf.min.js in central base.html

Resolve https://github.com/learningequality/ka-lite/issues/2013

Currently D3 based coach reports are non-functional on the central server. This fixes that by importing the widespread dependency module, sprintf, into the base template of the central server.

Fixes issue, and I would think is low risk.

@jamalex or @aronasorman - can you review and push to the server if it is satisfactory? @mikewray has suggested this is a blocker to demoing for deployments.