Add `variant_id` argument to `content_render()`

[toph-allen]

The new content_render() function should optionally support passing in variant_id to render a specific variant. This should probably return a VariantTask (right now it returns a ContentTask) but I think I'd like to change that class relationship to content. Not sure if that should be a part of this work or not.

Follow-on work from #283.