Add code highlighting option

This is a first stab at having a configurable code highlighting option,
similar to the MathJax rendering option. This change makes a checkbox
in the settings for code highlighting using the highlightjs.org
library.
What works: code highlighting in multi-user env is like I would
expect. single and anon(?) needs work

Things to resolve/consider:

• does the .IsCode test for code highlighting need to stay? At least

this and that should use the same version of the highlight.js lib.

• can the common templating part be 'included' somehow?
• the anon vs single-user vs multi-user code is not completely

clear (to me)

• bring js to local instead of cloudfare cdn (perhaps combine with

MathJax)