Instant Search: Adjust number of words/snippets shown

When on mobile we have less space to display results. We should add a param to the api to indicate if we need to do a smaller set of snippets. I don't think we want to explicitly set a number from the API since we may want to adjust it a bit on the fly.

Trace the Instant Search API and the mobile result-display path first, then identify where the current snippet or word count is selected. Define and implement a request parameter that lets mobile results use a smaller, adjustable snippet set, and verify that desktop behavior remains unchanged.

Written by the indexing model from the issue text.