Include grunt-digest in dev/prod builds

The digest task will output a JSON file into the public directory
which contains a mapping from filename to filename-cachebusted.

It will also *copy* all the assets in the public directory to their
cachebusted names.
Add cachebusting to spotlight.css

As a proof-of-concept, cachebust this one file. This involves converting
it to use underscore templates rather than mustache. Node.js templating
is painful.