[REFACTOR] Changed assessment demos to use questions api V2
[REFACTOR] Turned on options for items api (assess) demo and added idle timeout
[BUG] Repointing items api to prod
[POLISH] Added example spans on featuretypes demo page and codemirror.js for beautification.
[FEATURE] Added formula question demo.

NB This page links to the formula question draft page on the docs site.
The link will need to be changed when the draft is absorbed into the main
list of questions.
[FEATURE] Added "idle" timeout options to the override settings modal on Items API demo 
[FEATURE] Added stylesheet override to Items API Assess demo.
[POLISH] Removed question mark in label on items assess override modal.
[REFACTOR] Removed default custom css for items settings override. Updated contents of custom.css