Add word count to models

[mlissner]

We've occasionally thought it would be useful to have word count for every opinion. A user just asked me for it too. We should do this, with an index, and probably make it searchable too.

I'm on the fence about whether we should add this to the RECAP Documents. Maybe. I could see it being useful when searching documents, but we already have page_count in RECAP, so that's probably fine.

We can do this later, next time we do major schema updates.