(DNM) 926316 redesign search indexes

DO NOT MERGE (yet)

bug 926316 asks for a few search scenarios:

* CSS class names
* KumaScript macro names
* HTML attributes

This is my first stab at:

* Adding extractors for those scenarios, and creating new ES indexes for them
* Adding a primitive command syntax for searching `!kumascript {arg}`, `!classnames {arg}`, and `!attributes {arg}`

So, for a spot-check:

* Get some documents - ideally from a prod dump
* Reindex everything with `manage.py reindex`
* Search using the new commands - eg. `!kumascript cssref`, `!classnames external`, `!attributes href="http`

But, I don't quite think this is yet ready to merge, because it's my first time dealing with ES and I think it needs some additional eyes on how I'm handling the index types & etc.